What kind of samadhi is the Ven.Sariputta talking about here?

These are attainments on the treshold of Nibbana, they are an attainment of that nibbananirodha-principle which is the immediacy by which taints are destroyed.

• Emptiness, the signless, & the undirected are names for a state of concentration that lies on the threshold of Unbinding. They differ only in how they are approached. According to the commentary, they color one's first apprehension of Unbinding: a meditator who has been focusing on the theme of inconstancy will first apprehend Unbinding as signless; one who has been focusing on the theme of stress will first apprehend it as undirected; one who has been focusing on the theme of not-self will first apprehend it as emptiness.

These nibbananirodha attainments are talked about as path-fruition attainments, or as signless[themeless]/undirected/emptiness release depending on how the are approched, depending on how they are apprehended and whether any & what change is seen in one who emerges from them.

This attainment is the difference between a dhamma-follower and a stream-enterer who has directly seen the truth of cessation.

One can become absorbed for up to 7 days & 7 nights, dwelling sensitive to unalloyed pleasure of the Asoka [sorrowless] reality.

Now, I — without moving my body, without uttering a word — can dwell sensitive to unalloyed pleasure for a day and a night... for two days & nights... for three... four... five... six... seven days & nights. - MN 14

"And what, Ananda, is another pleasure more extreme & refined than that [of neither perception nor non perception]? There is the case where a monk, with the complete transcending of the dimension of neither perception nor non-perception, enters & remains in the cessation of perception & feeling. This is another pleasure more extreme & refined than that. Now it's possible, Ananda, that some wanderers of other persuasions might say, 'Gotama the contemplative speaks of the cessation of perception & feeling and yet describes it as pleasure. What is this? How can this be?' When they say that, they are to be told, 'It's not the case, friends, that the Blessed One describes only pleasant feeling as included under pleasure. Wherever pleasure is found, in whatever terms, the Blessed One describes it as pleasure.' - SN 36.19

Here, venerable sir, whenever we want, by completely surmounting the base of neither-perception-nor-non-perception, we enter upon and abide in the cessation of perception and feeling. And our taints are destroyed by our seeing with wisdom. - MN 31

There are, monks, three unskilled ways of thought: thoughts of lust, thoughts of ill-will, thoughts of hurting. And these three unskilled states disappear utterly in him whose heart is well established in the four foundations of mindfulness, or who practices concentration on the signless. - SN 22.80

Practicing signless release utterly destroys taints and that result [arahantship] is called 'the unprovoked release'.

"Passion is a making of themes, aversion a making of themes, delusion a making of themes. For a monk whose fermentations are ended these have been abandoned, their root destroyed, made like a palmyra stump, deprived of the conditions of development, not destined for future arising. To the extent that there are themeless awareness-releases, the unprovoked awareness-release is declared supreme. And that unprovoked awareness-release is empty of passion, empty of aversion, empty of delusion. - SN 41.7

Here some favorite texts from Therigatha

Four times, five, I ran amok from my dwelling, having gained no peace of awareness, my thoughts out of control. So I went to a trustworthy nun. She taught me the Dhamma: aggregates, sense spheres, & elements. Hearing the Dhamma, I did as she said. For seven days I sat in one spot, absorbed in rapture & bliss. On the eighth, I stretched out my legs, having burst the mass of darkness. -Thig 3.2

[I thought:] "Plowing the field with plows, sowing the ground with seed, supporting their wives & children, young men gather up wealth.

So why is it that I, consummate in virtue, a doer of the teacher's bidding, don't gain Unbinding? I'm not lazy or proud."

Washing my feet, I noticed the water.

And in watching it flow from high to low, my heart was composed like a fine thoroughbred steed.

Then taking a lamp, I entered the hut, checked the bedding, sat down on the bed. And taking a pin, I pulled out the wick: Like the flame's unbinding was the liberation of awareness. - Thig 5.10

This last line of verse is noteworthy to dhamma-nerds because liberation of awareness also occurs as 'awareness freed from consciousness' in Bahuna sutta.

Freed, dissociated, & released from ten things, Bahuna, the Tathagata dwells with limitless awareness. Which ten? Freed, dissociated, & released from form, the Tathagata dwells with limitless awareness. Freed, dissociated, & released from feeling... Freed, dissociated, & released from perception... Freed, dissociated, & released from fabrications... Freed, dissociated, & released from consciousness... Freed, dissociated, & released from birth... Freed, dissociated, & released from aging... Freed, dissociated, & released from death... Freed, dissociated, & released from stress... Freed, dissociated, & released from defilement, the Tathagata dwells with limitless awareness - AN 10.81

This seems to be a very baffling Sutta passage where he describes a seemingly contradictory state where one is in a special Samadhi beyond neither perception nor non perception but is still percipient. What is this Samadhi attainment called?

It does seem baffling because this state is only available to those who already attained one of the four Paths, according to the Commentary:

Mp identifies this with the concentration of fruition attainment (phalasamapattisamadhi). This attainment is not the fruition that occurs for a few moments immediately following the path, but a special meditative state accessible only to those who have already attained one of the four paths and its subsequent fruition. The attainment, as shown in this sutta, does not take any of the mundane, conditioned meditation objects as its support; its support is the unconditioned nibbana, experienced directly and immediately. The commentaries hold that this attainment is graded as fourfold according to the four stages of realization (from stream-entry to arahantship).

AN 10.7 appears to literally say the perception is of Nibbana (Unbinding).

But what, friend Sariputta, were you percipient of at that time?"

"'The cessation of becoming — Unbinding — the cessation of becoming — Unbinding': One perception arose in me, friend Ananda, as another perception ceased. Just as in a blazing woodchip fire, one flame arises as another flame ceases, even so, 'The cessation of becoming — Unbinding — the cessation of becoming — Unbinding': One perception arose in me as another one ceased. I was percipient at that time of 'The cessation of becoming — Unbinding.'"

Ud 8.1 defines Nibbana as follows:

There is, bhikkhus, that base [ayatana; sense object] where there is no earth, no water, no fire, no air; no base consisting of the infinity of space, no base consisting of the infinity of consciousness, no base consisting of nothingness, no base consisting of neither-perception-nor-non-perception; neither this world nor another world nor both; neither sun nor moon. Here, bhikkhus, I say there is no coming, no going, no staying, no deceasing, no uprising. Not fixed, not movable, it has no support. Just this is the end of suffering.