That wasn't the original derailer anyway.

I'd replace it with another Simplex. They're made to fit a 28mm French seat tube. Some others will slip. I believe that the correct one for that bike is a Super Competition. I may have the original from my PX10. If I do, and it's still functional you can have it if you want it.

I just went out to the shop and dug it out. The plastic is cracked. You don't want it. There was a NOS one on eBay last week. I rarely see them.