Just in case someone else runs into similar issue (broken lens) -- perhaps the whole thread should now rather be in the repair section:
> The 427A meter this appears to be is the same form factor as the movement for my 419A
On a flea market I acquired a MCM from what might have been a HP400EL or 400FL. Same form factor, but those AC meters don't have a mechanical null adjustment. The front face seems to be prepared to take such, the hole is just covered by some black rubbery stuff. I was able to transfer the disc with the pin grabbing the null point adjustment, but I can't get the plastic button (head of screw?) in the front out of the broken meter. Oh well, works kinda, sorta without.
> I had a look and to me the MCM seems sealed (I found four brass screws, but after removing those, nothing comes apart).
Not sure what happened there, but now after removing those four brass screws in the back of the MCM, the front face with lens comes off fairly easily (held in place only by the QA sticker/seal on the bottom).
Just replacing the glass window itself would be a bit of a challenge, as it seems to be glued in place. Don't know how to soften or remove that glue. As it is, the glass would rather shatter than separating from the plastic frame.