Ok, the HM does not have a cover. :whistling:
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:
while everything was fresh and should be of help.
Disregard the previous arm adjustment statement.
The camera came "to me" with a 135mm lens but I do have a 127mm lens. Thanks for the information.
Due to production tolerances, which were wider in the past, a lens is rarely its marked focal length. Some lens are designed with shorter than their marked focal length back focus so that they will work on more cameras.
A 127mm may be optically 123mm to 130mm while a 135mm may be 132mm to 138mm. There may be enough adjustment range of the HM RF that a marked shorter focal length RF will adjust in on a lens that is marked with a longer focal length but optically measures close to the upper limit of the shorter focal length.
I have a 135 Xenar that infinity focuses a good 5 or 6 millimeters closer than my 135 Optar which has a factory cam for 136.4mm and is spot on wide open. I also have a 127mm Ektar so I plan to see if the HM RF on the Anny. Speed will adjust to the Xenar one of these days.
I was able to adjust the HMRF on the Anny to the 135 Xenar. Its a hair soft ( 1/3 stop) at 4 feet and sharp from 6 feet to infinity.