Assistance needed dating Turmon (1 Viewer)

[Ignatius]

The Zeiss Turmon (1921-1990) was available in several 'series', most of them with serial numbers either above the logo on the brass objective prism mounting plate (at least until the 1940s) or on the then plastic ocular body (1950s), but it seems that some had no serial numbers at all. I have absolutely no idea why and when when that started/ended, but I believe it was concurrent with the VEB Carl Zeiss Jena Q1 export models.
Does anyone here have more information on Turmons in general, and specifically dating them?

Thank you.

 

[Ignatius]

Ok. I have worked out myself roughly how old my Turmon is.

What I cannot understand is why it will not focus to infinity, ie. further than about 10-15 meters. See here and here.