Since neither 66/99 typically relates to 44 caliber, in such as some european bore or gauge indicator. I can only assume that its probably just an assembly number of some sorts. Molds normally arent found bearing serial numbers, but frequently found with makers stamp such as Colt, or Mass Arms etc. These were usually included with a boxed pistol/revolver as part of a kit, which is typical of a civilian or private purchased item. Military normally issued pre-packaged cartridges.

Frederick