I suspect a typo. When you go to Buffalo Arm's referenced cowboy ammo section, there are no listings for .45 ACP, contrary to what they would have you believe. Cowboy ammo in .45 Colt is common; but, I've not heard of cowboy loads in .45 ACP. The .45 Colt started as a black powder cartridge, the .45 ACP originated as a smokeless load.
If the cylinder was made to accommodate .45 ACP, one would expect to see it cut to accept moon clips as is done in other .45 ACP revolvers.