Weird 'Sight' on a Long Colt?

I have been cleaning up a Foundry mounted U.S. Cavalryman for my Old West project, but I stalled when I reached his Colt.

It looks like there is a sight 2/3 down the barrel, instead of near the muzzle, and this baffles me a bit.

I wonder if any cavalry revolver actually looked like this, or if it is just the result of a casting vent on an old mold?

I would be much obliged if any of my readers can shed some light on this for me, as I haven't been able to find anything on the matter...