I really want to keep the Derby going as is
probably just buy 127
but I will fudge up an adapter to use it on leica mount when time permits
really wish I had a lathe etc but then that'd be something else to learn

I stuck the negs under my new D5 w/aristo and with a grain focuser read the 3 bar target

all I can say is that with the Foth at 3.5 and very carefully scale focused -no marking for 7 1/3 feet on the derby
The Derby actually seems to resolve more than the Zuiko 50 1.8 at 5.6

The Zuiko LOOKS sharper and I figure it's due to contrast
The Foth is much less contrasty a lens
BUT the Foth went 1 1/3 - 2! more 3-Bar "squares" than did the Zuiko before getting very mushy/unreadable

? I read that as being pretty damn good.

The Industar rigid 50 3.5 ugly stick is a contrasty lens reading just about equal with the Zuiko
Maybe more contrast than the zuiko to make up for resolution, i dunno

Minolta 7s Rokkor less contrasty and resolves just about 1/2-1 square 3-bar less

still have to redo Retina f2 Xenon but in 4x6 landscape prints looked "just as good" as the other lenses