Most say 38 special on the barrel
Production date is 1977 for serial numbers: 19201M-59999M. Don't know what the X is.
Any commercial 38 special ammo can be used in a firearm marked 38 special caliber. (Remington,Federal,CCI,Fiocchi,etc.) The Colt serial number guide that I found on the internet says Colt Detective Specials with an "M"after the serial number started with the year 1975. The gun should have a shrouded extractor rod.