Bell & Howell/Canon FD 135mm f/2.5 S.C. breech mount
Canon F-1 w/ mirror and diaphragm prefire

Vignetting: B+ @ f/2.5, A thereafter
Distortion: none

Aperture Center Corner
f/2.5*   B-     B-
f/2.5    B-     B-
f/4      B      B
f/5.6    B      B
f/8      B-     B-
f/11     B      B
f/16     B      B-
f/22     B+     B-

Notes:
moderate contrast at f/2.5 (with [*] and without Canon Skylight filter);
moderately high contrast at f/4 through f/22.
This lens was marketed in the USA in the earliest days of the Canon FD lensline.