Bren serial numbers.

[Peter Laidler]

D3627 is very probably within the last of the true Mk1's or very early into the first of the of 6 transitional stages from Mk1 to Mk1A. In that the crown/Enfield/1940 on the right hand side (which took a whacking 12 minutes to engrave incidentally) was eliminated for a clearer Mk1/DE/1940 on the left hand side. You'll have to tell us where your markings are for certain. D3627 is probably mid-ish 1940 but the 1940production went up to the F-51xx numbers
Butt slide H8330 is a little way into the start of true Mk1A production. The Mk1A is a simplified Mk1. Britain never produced a Mk1m. This H8338 number should not be stencilled. It should be engraved (or stamped) at the rear of the butt slide above the small of the butt.
X3402 is an Enfield Mk1A from close to the end of 1942 production. I'd say, October ish