score:14
You were given a code 3 leave to enter instead of a 5N. It means the IO did not have sufficient grounds to refuse you entry but had some reservations and hence 'landed' you on code 3. This means that the next IO will be aware of this since you were not given the usual code 5N stamp. The NWA 45 followed by three digits is derived from your landing card. This is your arrival number. The IO would have made notes on the back of the landing card for future reference setting out his suspicions.
BTW you were NOT given temporary admission (in the IS96 sense) as this term is used erroneously in the previous post. You were given 'Leave to Enter' since you were landed.
Upvote:3
As Gayot Fow explained in his very detailed answer to your other question, you were not refused entry: this stamp is a standard "leave to enter" stamp, which means you were granted entry to the UK. These are commonly given to everybody who is allowed to enter, but doesn't otherwise have right to abode in the UK.
The code up top is likely there to identify some combination of where you got this and who gave it.