Thanks to all who helped figure out what model this rear derailleur (probably) is -- it was just killing me not to know, and folks were very generous with their knowledge. This derailleur is a vertical parallellogram rear mech with alloy upper/lower pivot castings w/ Simplex logo, steel parallellogram legs w/ a Delrin infill on the back, and short cage. It has a mounting ear/claw, and a bronze Simplex badge w/ black lettering. No legible model number stampings. This critter likely went on mid-range French bikes of the 70's, is well-regarded but not the drool-after LJ all-alloy model. If you have a bike that needs this derailleur (please let's work toward completing our bikes), it's yours for postage plus the $5 I paid for it about 20 years ago. I never used it, so really don't know that everything's OK. Condition: used but not worn out, with scratches here and there, but doesn't appear to have worn tolerances, slop or signs of abuse. Jockey/tension wheels will need lubing.
Pls contact me offlist and thanks again to all!
Reid Fisher
San Martin, California