Leather Subarmalis

[M. CVRIVS ALEXANDER]

There's a 4th-C AD primary source reference to by-then-ancient Roman soldiers using a felt undergarment

It seems to me that the De rebus bellicis 15 is unclear what time in history is meant by the "antiquity" that invented the thoracomachus - it could be fairly recent or quite distant. That is, it is impossible to know when in the past, according to this source, the thoracomachus was in use. Moreover, it is also unclear, IMO, whether it was actually in use in the Roman army prior to the writing of the work or the author is rather advertising its usefulness.

Another text that may be relevant to the issue of the subarmalis is Caesar's BC III.44:

When Caesar attempted to gain any place, though Pompey had resolved not to oppose him with his whole force, or to come to a general engagement, yet he detached to particular places slingers and archers, with which his army abounded, and several of our men were wounded, and filled with great dread of the arrows; and almost all the soldiers made coats or coverings for themselves of hair cloths, tarpaulins, or raw hides to defend them against the weapons.

According to the passage, a protective undergarment was either absent or ineffective in the situation described.