A sidelight. After c. 1874, the Army never issued the .45 [Long] Colt. All army issue ammunition came from Frankford Arsenal and they made only the ".45 Army" aka ".45 Government" the short or Schofield length case. Even when the Army adopted the Model 1909, they issued only the Model 1909 ammunition, even though Colt had actually chambered the gun (a New Service with Army markings) for the .45 Colt.
To clear up any confusion (or make it worse), the R in the revolver ammo headstamp means "Revolver" just as Bob says, but the R in .45-70 headstamps means "Reloading."