It’s looking increasingly likely that this is the missing Mond lens. It’s aperture is 17 cm and the focal length seems to be around 120cm when bringing a ceiling light into focus on a screen, which agrees with the original specification in NLO Handbook 1935. We have also found some numbers inscribed into the rim of the lens carrier but cannot read them clearly enough yet. The lens also fits comfortably into the front of the telescope tube. It does look like a petzval lens as you suggest Dennis, so my only query is was the original lens a petzval? We also believe that the accompanying guidescope is also original to the Mond. We believe it is the bottom section of a longer instrument which you see in the above photograph. I thought it just had its lens missing but now believe another longer brass tube holding the objective lens was located over the end of this short one.