Come to think off it, your are right. I was not thinking. How in the world would moon clips be used in a single action revolver that has to be loaded one round at a time through the loading gate?
Sorry, I wasn't thinking.
I have no idea why that groove is there at the rear of the cylinder.
The simple fact is, I have always fired 45 Colt ammo through my old Blackhawk.
In fact, when I bought it way back in 1975 I didn't even want the extra cylinder.
I used to be able to buy relatively inexpensive 45 Colt reloads, so that is what I was shooting in my Blackhawk all those years ago.
Then I started reloading my own 45 Colt ammo, so there was no need for me to be buying 45 ACP.
It was actually many years before I ever used the 45 ACP cylinder, it has been sitting in the little red bag for over 40 years, and has hardly any wear on it at all.
Sorry, before anybody asks, the ACP cylinder is not for sale.