The 1938 is the year of manufacture. S/243 is Mauser's second rifle plant in Borsigwalde that began production in 1935 with the manufacturing code S/243 G. Your 1938 was an earlier 38 manfacture after dropping the K (1934) and G (1935) as year codes. This began in 1936 with the year stamped on the receiver instead of the code. The S code was also dropped latter in 1938 and was just designated as 243 for Mauser Werke AG. Borsigwalde. Same factory, just different codes.
The 280 on the barrel and receiver is correct and I believe the serial # of 3092 a, means your rifle was the 13,091 rifle produced that year. Serial # started at 1 or 0001 and ran to 9999. that started over at 1 or 0001 with the alphabet suffix, a, b,c, etc.
Not sure about the extractor and collar but the #8589 on bolt handle and #1771 on bolt sleeve and safety lever show these are mismatched to the receiver and barrel. All serial numbers should be 3092 with the smaller parts just having the last two numbers of 92. It seems all WaA should be 280 with the exception of the safety, which is already mismatched, as 280/37. Firing proofs on the barrel and left side of the receiver should be Weimar-style eagles, not Nazi style. Waffenamts should be both Weimar-and Nazi-style eagles. Could have either a walnut or laminate stock with a flat buttplate.
Hope this helps.:)
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