A brass viewfinder marked T. T. & H. (we know what that is), and it has what I can approximate as a ratio of 20 x 28. The finder can be rotated to vertical, horizontal. I am reluctant to disassemble it to make an exact measurement.

In my mind (a poor measure) it appears to give a nominal 'normal' perspective for the diagonal.

Any clues to what I have would be greatly appreciated.