wow!! As luck would have it @reversalcine noticed an 80mm lens for my Kodak Retina IIIc for sale on Trademe! Got it for $75 and picked it up from a charity store two suburbs away lol… I had to RTFM how to swap lenses over, but worked it out – check how tiny the 50mm F2 lens is in comparison – thats it sitting beside the camera with the 80mms lens mounted! Test roll of Kodak TriX400 loaded…

Important part as text:
Focusing the 80mm lens:
Before exposing with the telephoto lens, measure the distance of the subject with the rangefinder of your RETINA, and read off the distance against the index (15) on the focusing scale (4).

Then turn the camera towards you and transfer the measured value to the focusing scale for the telephoto lens (37) marked in white figures on black.

To do this turn the focusing knob until the appropriate distance on the telephoto scale is opposite T-index mark (37 a).

Do not, however, use the black ring (39) on the telephoto lens itself for setting the distances.
This ring is part of the depth of field indicator and serves only to show the depth of field available.

Thanks to Butkus website for archiving this & many other vintage camera manuals