I have two Remington A3s, one sadly deactivated the other a very good 'shooter'.
The Serial numbers for those are:
3976XXX Remington Barrel marked 3-43
and
4143XXX again Remington Barrel marked 12-43
Both have all the correct Arsenal marks and I'm pretty sure that the barrels are original fit. Now the thing is, according to this site:
http://oldguns.net/sn_php/milmods.htm
both have a manufacturing Year shown as '42.
Anybody guess why there might be a discrepancy between the manufacturing year and the barrel stamps.

