Dan, the viewing lens is fairly well calibrated to the taking lens, in that I get the centre sharp when I need to. The examples were simply deliberately back focused to emphasise the relation of the centre to the edges.
I have removed and stored the thin washer/shim from the lens. The front element is seated securely. I can't draw any conclusions from my test image, and I will have to shoot a few rolls and see. By the test image HERE,
It seems it may be slightly better now, or I may be imagining things. Differences between centre and edges are still visible at f/5.6, and the differences appear to be symmetrical.
(the mountain's edge is further away on the right than the focused centre) The skew light pole is about a meter behind the white wall on the left.