When I don't have the original rifle in front of me, and I don't have images of every detail, I wing it by using a motif from another part of the rifle.
It might be a simplification of the patchbox finial. Or a repeat of the entry pipe carving, or an echo of the sideplate. In this way, it's not going to stand out as stylistically different from the rest of the gun.
Does this help?