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I have yet to buy them, so best guess, is the eye is to tow the gun, the other bit to go under the barrel for elevation/depression for aiming , I struggle to see what else it could be.
It looks to me to be a peg for the hole at the back of the gun carriage, where handspikes would be used to help move the gun.
It doesnt look like something that would be involved in attaching to a limber & it is not an elevation screw for sure.
Ah have just seen PSR have updated their entry for these guns, and yes it is indeed for a handspike.