Here are some photos or original 1860 Army revolvers. It looks like the groove was a standard feature.
It's my guess that it was put there to help the shooter guide the cap onto the nipple with their fingers.
Unless someone can let me know that I'm wrong, they didn't have cappers back in the 1860s. AFAIK, they put the caps on with their fingers.
I have read that Colt made cappers but can't fine where I read it or any pictures. But if Colt could invent the revolver I am sure he could come up with a capper.