Ok, the HM does not have a cover.

The lower mirror should move turning down/inward as the arm moves inward; out/up as the arm moves out.
The #4 HM is for a 127mm lens. The letter is production code.
I posted some info on a HM 4C a while back:
http://www.graflex.org/helpboard/vie...ght=hugo+meyer
while everything was fresh and should be of help.
Disregard the previous arm adjustment statement.