Upvote:7
This order is quite old and previously called Hospitaller. The earlier designs were much simpler crosses and not really "arrow-like". The design is quite varied over the medieval period and ranged from a plain Latin cross to the 'modern' design.
Obsidionis Rhodie Urbis Descriptio, Bibliotheque Nationale, Ms. Lat. 6067, f37v, Paris, France.
While there was an order that has probably a very similar design, it was probably not what is requested here:
As the members of that order ultimately ended up in the Knights Templar.
If we look at either the now older depictions of members of the Hospitaller order it becomes clear that the association for a hammer is much easier when looking at the flag, ancient and for the Maltese as well, when just one "arm" is taken away:
But there may well be such a half cross more resembling the arrow heads:
"This anonymous but thoughtful-looking young member of the order was painted by Francesco Francia in the early 16th century. (National Gallery, London, England) (David Nicolle: "Knight Hospitaller. 1306–1565", Warrior, Osprey, 2001.) [Note that unless a better reproduction comes along, there remain some doubts whether this is not coverd up by the overcoat?]
As for the Spanish martillo as a term for that variant of the cross: I only ever found English literature referring to half-brothers (confratres) wearing "half-crosses" (example).
One of those, online, even has an interesting play on that "cross image" in heraldry:
Arms of Philippe de Villiers de l'Isle-Adam as bailli; Rhodes.
Bailliffs (or Baillis) were the heads of the Tongues and holders of the great offices of the order. Since the 15th century they bear a chief with the arms of the order. Miniatures in Caoursin's manuscript show the Baillis in session around the Grand Master, each holding a string of beads with a fiocco at the end; also, the tombstone of the French Bailli Montmirel in the Archaeological Museum, Rhodes, shows two such strings placed on both sides of his coat of arms.
According to the descriptions the early designs of the Hospitaller were not only often quite similar to the design of the Teutonic knights, their statutes also were similar in allowing donats and confratres only wearing the half-cross, not the full one. Going from there, an old seal of the city of Allenstein (Olsztyn) displays such a half-cross (of the Teutonic knights, which nicely illustrates why one might see a "hammer":