That is a late S&L ek1 as Peter correctly remarked. Notice the ring at the bottom of the frame. The same frame could be used for the ek2 and that round mark is where the ek2 loop is attached.
Hello.
I think i have found another maker. I first thought this would be a KQ for Klein & Quenzer.
I think its KO for Kerbach & Oesterhelt Dresden
The cross posted here is a 1957 version.
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