I've just put my recently acquired Omnichrome 643 on the bench to start re-aligning the optics and noticed it's rather unusual in that the OC is at the cathode end of the tube rather than the anode. This was obviously an OEM head (643-LICA), but really not sure why they needed the output beam at the 'wrong' end. I don't know if it's of any benefit to rebuild it the 'right way round' or just leave it as is; the mirror alignment is already completely lost, so it's going to need completely realigning whatever I do.