105 was the mfg "norm" for both taking and enlarging lenses per 6x9. For example, Both Fuji and Nikon
make 105 plastmats, and I personally own one of the relatively rare 105 Nikkor M tessars. But what is
"normal" to your personal way of looking at things is a completely different issue. I mostly use 85mm
with my Nikon 35mm camera, a 165 with 6x7, 180 or 200 with 6x9, 250 with 4X5, 450 with 8X10;
in other words, all these have an analogous angle of view which is natural for me. Someone else might
have a completely different notion of first base. Strictly defined, normal is equal to the diagonal of the
film; but who gives a damn?