Thanks for the welcome!

Bowtie's pictures illustrate the external differences perfectly. I have different rear caps on the rough units though. Also I ( mistakenly?) reversed the dating. I took a fait little stamp on the side of the polepiece for a date code. That suggested 60 something for the rough ones (right in the pic above) and 70 something for the other ones.

I am still curious about the weight difference. Is is due to a smaller magnet, less casing material or both?

The lighter, smaller and smooth units do seem to have a little less output, yet sound fine nevertheless.