If you would give us a serial number we might be able to answer your question. Otherwise, all we can say is "sometime between 1911 and this afternoon".
'ACP' means Automatic Colt Pistol. A 1911, for example, would chamber a .45 ACP cartridge.
45 ACP
You will need a gunsmith to find out.
Any manual for a Colt 1911 will do.
Proofhouse.com has Colt serial numbers you can look up
Answer The Colt .45 caliber automatic pistol was accepted as the standard side arm of the US Army and became identified as the Model 1911 Pistol. The bullet developed for use in this new design of automatic pistol was called the .45 ACP or the .45 Automatic Colt Pistol cartridge. It stands for "Automatic Colt Pistol" because it was adopted for the Colt 1911 only by the US army.You get many variants the most prolific being the .45 GAP for glock handguns.It's patented I believe.
about 1978
1970
don't find any 1911 45's serial number ending with a letter. ???
Manufactured in 1978.
ACP and auto are used to say the same thing. Automatic Colt Pistol.
Colt Firearms ((named for its founder, Samuel Colt) was one of the firms that made the US Military Pistol, Cal .45 ACP, Model 1911. It was adopted by the US Government in the year 1911.