Tyson,
The casing of the diff on my '69 FHC is stamped "A69 5093". I suspect that, probably as you suggested, the # is a manufacturing coding (maybe A69 = 1st week of 1969?). I seriously doubt that the diff manufacturer Salisbury would know what car the diff will be fitted to nor would the engine serial # be any more important than the serial # stamped on your transmission casing.
What is important is the tag attached with a bolt to the rear diff cover "46/13" = 3.54:1.