I have two Zeiss 3-9 X36 Diavari-C scopes and they are so marked along with 'West Germany'. One is a 5 digit serial number: 41XXX and the other is 6 digits 650xxx. Both are the armored objective variety and each sports the same duplex reticle (duplex).

The 'newer' one (higher serial number) is about 3/4" longer and it appears the addition length is in the tube forward of the adjustments.

Might Zeiss have lengthed the tube at one point to fit longer actions? I've also read that there were two types of coatings used on the lenses. Was there a serial number where the coatings changed.

You can tell the difference by the markings on the ocular. The older versions had the better T* coatings and are thus marked. The later model was more similar to the Conquest line and is marked MC for multi-coatings.

BT, did they make a long and short version? Or did the later ones all have a longer tube (for longer actions?).

I think they did make them in two lengths, with the shorter versions being used on Kipplaufs, BBFs and Drillings. They also made them with a trapezoid rail underneath for directly mounting on claw mounts.

I have a zeiss 6x42 of 1974 vintage on my 7x65r. It’s optics are still brilliant - as good as my Leica 1.5-10x42 which is only four years old.