I shouldinclude more information on the revision numbers: The revision 12 bolt was used from early (April approx) 1942 to late 1944, there are many heat lot steel codes also used on these bolts, your bolt is correct. The revision 12 follower was used from about 2.1 million to about 3.6 million (there is overlap). The milled and stamped trigger guards overlap each other during the mid 1944 time period. The Type 2 and Type 3 lock bar rear sights also overlap during 1944, also during 1944 the long and short follower rod and gas cylinder lock screws will overlap as the grenade launchers were being issued. The checkering pattern on the buttplates also changed from the 1941-42 type to the late war period.
The gas cylinder changed from the early narrow front sight base to the wide base during the war, by 1943 the front sight screw cover was deleted. There is alot more but this
should highlight the changes