Please help identify the lens

[D. P.]

Please help identify the lens.
There are no inscriptions.
F/2.8, focal length approximately 100-105-110 mm, 10-blade aperture, weight about 500 g, T2-M42.

[calvin83]

Interesting find! I can't remember if I have ever see a pre-set 100mm? 2.8 lens with a pre-set ring below the focus ring.

[kypfer]

I've got a 135mm f/2.8 "Fontron" with very similar mechanical construction, 10-blade aperture, 332gm with a 58mm filter thread, albeit a slightly different font for the lettering.
The front end of mine slides forward about 1cm to create a basic lens hood.
Mine arrived as part of a job lot with an Exa camera that was fitted with a Tessar I wanted.
I've yet to try the Fontron, but they do say "there's no such thing as a bad 135mm"