Quick Question: I acquired at a quite reasonable cost a 180 (pardon, 18cm) lens for the C220. I noticed the original pictures with it were seriously off, and upon inspection saw that someone had cleaned and reassembled the taking lens rear element upside down.
The question is, given this problem, and the possibility of other parts having been swapped, are the Taking and Viewing lenses the same on an 18cm f4.5 chrome lens? The look alike, weigh the same, and the elements appear identical when disassembled (i.e. use them as a magnifyer and check the edges, check the focus on a light at infinity, etc). The spacer was also a dual ring (figure 8 shape), designed to shim both lenses equally.
When I swapped the rear units before deciding to reverse element 4, I could get an image on the ground-glass which was identical to the off-pictures.
Btw, in follow up to an old post, the C220 w/ 65mm black + 18cm chrome weighes less than 1/2 a Stone. It remains the hiking camera.
Although I wrote an article for Bob Monaghan's site obout the Sekor lenses for the Mamiya TLR, I do not consider myself an expert.
The 180mm Sekor is not a true telephoto lens but resembles the old 1920s Ernostars by Bertele. But there are significant differences including: the cemented elements in the first group are reversed, the second element of the Ernostar is a cemented doublet, and the final element of the Seiko is a planar meniscus.
Bertele designed the Ernostars when working for the Ernemann Company. When the company was taken over by Zeiss Ikon, Bertele began work on an improved Ernostar design. Later still Bertele used the improved design as a basis for the famous Sonnars. So although the Seiko can trace a pedigree with the Sonnars, they have very little in common.
I have disassembles my 15cm and you are correct, it is easy to reverse the elements. From what you wrote, it seems that you have correctly diagnosed and solved the problem. Yes other elements could be reversed, but that rear element is the most probable. Your reverse-logic method, deliberately producing the error is a good play. I don't think you should have any more problems.
If I can be of further help please feel free to email. truly, dr bob.