I don't know an exact date, but I suspect it's when Gaston decided he didn't want to advertise a competitor on his product. The .45 ACP and .357 SIG also now read just .45 and .357. You will note,though that the .45GAP is proudly displayed (and also, I suppose, to keep people from trying to load .45 ACP in it )
There doesn't seem to be a pattern. My G-23 s/n SLxxx is marked .40 SW, while a friend of mine has an older G-23 s/n RPxxx that is only marked .40. Frosty
Not that it matters much, but the 45 is marked .45 AUTO, not just 45. You're right, by taking off the .45acp, they removed Colt's name.
I thought it had something to do with Smith & Wesson not being able to claim .40 anymore. I dont know where I heard that, but thats what I heard.;g