Pretty consistently on the Roman Republican Series the attribute of Saturn is identified as the harpa. In fact the logic is a little circular, if its Saturn it must be the harpa and if there is a harpa with a bearded divinity it must be Saturn.
The harpa is a tool otherwise usually associated with Perseus who used it to cut off the head of Medusa and it is a regular numismatic symbol like Heracles’ club, though not precisely as common. Here they are together:
My problem looking at the Saturns is that the later ones clearly have the traditional harpa on them. See above, as well as this slightly earlier type:
But the object behind the heads of the divinity on all of these is clearly something different:
Whatever this implement is it clearly has ‘teeth’ and is a single curved piece, not a blade with a hook or prong, such as we associate with the word harpa. Of course, at its most basic the word just means sickle or curved blade in Greek, the general equivalent of falx, falxis in Latin. And yet, surely we can come up with some vocabulary discuss the shift in attributes of Saturn without rounding them all down to a generalization. The earlier Saturn is clearly associated with something other than the tool of Perseus with which he is bestowed in the imperatorial period. What is that earlier tool? At first I saw the teeth and shape quite literally as a jawbone. There are jawbones on some Greek coins, but they don’t look much like the tool:
My guess is that its some piece of agricultural equipment probably for reaping and that it has a specific name. At some point Saturn’s agricultural associations perhaps became a little less critical and his attribute took on a more mythological, apotropaic form.
In the interests of completeness I should also mention that there is this other early coin with Saturn not as an obverse type but as a reverse type. If I’m right, the tool in the hand should be be the toothed one, not the Perseus-style harpa, but frankly I’ve not yet seen one clear enough in that detail to say it looks like much of anything specific:
Saturn as an agricultural god of reaping would certainly have resonance for a number of these earlier moneyers, to say nothing of agrarian issues generally in the Republic.