If I understand your post, this is for a FSU rangefinder. So a lens from Fedka should work well. If it is for an actual Leica camera, it will likely need to be shimmed for optimum focus wide open, by a willing and able technician.
Fedka also has nice lens shades to fit this lens (tilted/vented 40.5mm).
I have had good support from Fedka. If there is a problem, they will accept returns within a given time frame.