But if they were used for pay we should have some sort of documentation. There would be mention of them in diaries, ledgers or artwork just like most every other aspect of Roman military life. There would be a story about one being lost on campaign, resulting in pay errors. There would a few on a mural depicting an accountant's or officer's daily life.
I think they are something more mundane. Pocket candle holder seems plausible. But the flip side is that with mundane items we should see greater diversity across the empire. The fact that we don't suggest a tie to something universal like religion or politics. Maybe they were a calling card, proof that the bearer was a member of a particular group or cult.
AFAIK, we don’t have a lot of “daily life” text from early Roman times. We have architecture and stone/tile art, from which we learn a lot about culture. And we have major milestones where those justified documentation. “Soldiers: how you get paid” is not likely in any of what we have. And it may not have been written; literacy was not high.
I think they are something more mundane. Pocket candle holder seems plausible. But the flip side is that with mundane items we should see greater diversity across the empire. The fact that we don't suggest a tie to something universal like religion or politics. Maybe they were a calling card, proof that the bearer was a member of a particular group or cult.