Pre-assemble the M2Hb custom airsoft gun top
rear

I have found
assembly simpler if
I pre-assemble the
M2Hb custom airsoft gun top rear panel M to the upper
side trims C and D. If you do this, then the resulting part
will make aligning the two large receiver plates much
easier to do. Holes are NOT marked on the receiver plates, and
it is best to match-drill them to this finished part for
best alignment. I usually glue this part up and make sure it
is aligned well before making any other parts for my Browning Machine
gun.
The M2HB/fifty cal body side plates are identical, but
mirrored when assembled:

50 caliber Left-hand sideplate.