Geezer here, concerning round stirrups. When I first went to look at the Padre Island crossbow bits back in 1994, it was clear we had a lightweight Spanish crossbow, much like the ones in the armory at Malta. That's a very common style. So we were reasonably sure our crossbow remains had once featured a very small nose-ring at the fore-end. Question: was the nose-ring strongly enough attached to fit a cocking lever to the ring, and could the crossbow be hung up on a hook to span as suggested above. Examination of the Padre Island bow indicated the ring was probably driven hot into a pre-drlled hole, so it would fit pretty tightly, but in fact, the only thing that could have kept it in place beyond the fit was a Very Small pin (no more than 1/8 in. dia) fitted underneath the typical cheek-plates that surround the prod and reinforce the fore-in. The pin was long gone, but the hole was there, and my friends it was positively dinky. If you tried to span that prod with the help of the nose-ring, the ring was going to end up flying across the ship. So in that case, the round ring was clearly intended for hanging the bow up on the wall or Maybe a saddle. Some later, 17th century clap-lock stocks use a half-round loop fitted to the fore-end of the stock with a solid pin, that could possibly be used as an anchor-point for a 'wippe' type push-lever.
Other bows, like the beautiful Maximilian I bows in Vienna, are clearly meant to be spanned with a Spanish-style cranequin. In that case there IS a stirrup, but it's much too small for normal human feet. Perhaps if Maximilian I was a Satyr, with very narrow goat-feet, he could have put a foot in there. The stirrup (not round, but rather D shaped) could have been used for hanging up the bow, but Maximilian-type bows often have a separate hanging hook on the bottom of the fore-end. So I suppose the jury is still out on whether That stirrup is intended to be functional. So the question about round stirrups remains open... cool. Geezer