The User's Review: Two 55mm F2.8 1:1 Macro Lenses,Branded By Panagor and Vivitar

Probably manufactured in the same factory in Japan, these two lenses are almost identical, except the engraved marks on the surfaces of the lenses.

The Panagor has the magnification scale on the rear side of focus ring, while the Vivitar put the scale on its barrel. On the Panagor, it marks PMC (Panagor's Multi-Coated?). The coating colors, green on front element and yellow on rear glass, are lighter on the Panagor. I am not sure, this is because of the PMC or different aging processes between the two lenses.

The Panagor lens shown here has a MD mount while the Vivitar lens owns a M/SR mount and a screw on its aperture ring (in the last image below). A small metal 'leg' might be mounted at the screw site to convert the Vivitar into a MD mount.