Originally Posted by
CV-6
Simplex LJ and SLJ would be period correct but pricey. Suntour V-Luxe shifts well but would require notching the derailleur hanger for it to work. I cannot tell from your photo if you have a hanger or claw so that may not be an issue for you.
A super LJ would be expensive, but there is nothing special about an LJ. Lucien Juy's initials are on every Simplex derailer I've ever seen.
I'd look for an SX610 with a claw mount. Prices have gone up, but I have five and I didn't pay more than $20 for any of them. You have to be patient to get a good price.