Here's my contribution to the 'what the fuck are these things' guesses:
Its a customizable, weighted D12.
You stick the coins into the slots, then do some kind of gambling involving dice rolls.
Part of the game is that as coins are added or removed, the weighting changes.
Hell, it could be that you take turns tossing the thing till its full of coins, thrower has to fill the hollow upward facing hole, as a way of anteing up, then just toss it again untill its empty, thrower takes out the upward facing coin, everyone takes a drink when it lands with a hollow slot face up.
The educated, literate people probably wouldn't bother to write down the exact details of a low class gambling game, when literacy is rare and scrolls/books are expensive.
The things have reportedly often been found in cabinets and drawers alongside coins.
They vary in size... maybe some of the rather large ones could be commissioned as not a practical gambling die, but as an exxageratedly large one, as a trophy or ornament, like modern mall ninja swords or funko pops or something.
Possibly they could also serve a practical purpose for normal people and coin minters to check that their coins are properly sized.
Any... Roman numismatists here that can sanity check this, in terms of coin sizes?