I have a Steyr-Mannlicher Model M "Professional" (black plastic stock) with a date code of "BTH" Based on the chart (see image), I interpret the proof date to be April, 1986. This is a Canadian rifle.

However, I have another one of these same rifles (brown plastic stock) with a date code of "280" ...this one seems to be an American rifle imported indirectly into Canada through the USA . On the left side of the barrel is stamped "Steyr Grand Rapids, MI". (The Canadian rifle has no importer stamp.) Did the Americans have their own date code or is this just an earlier "numbers only" code system?

Should I interpret "280" to mean a proof date of Feb. 1980?

The "American rifle" also has a stamp on the right side of the barrel; "ÖNORM M3170 STLF3". This is apparently a type of Austrian standard rifle barrel steel. This stamp does not appear on the other apparently later rifle.