I rarely find one of this type that hasn't been damaged on removal.
Seems that few bike mechanics know not to turn the bolt to remove these, and I've probably damaged at least one Simplex and Huret derailer this way finding out the hard way.

When the flats in the washer are completely gone, one can still easily install the derailer to the dropout and then tension the spring by turning the hollow pivot bolt clockwise while tightening the male-threaded mounting bolt from the back. This is actually an advantage over a new derailer in that the B-tension can additionally be fine-tuned to suit specific size freewheels, whereas the un-damaged Simplex derailer requires fiddling with the cage A-pivot shaft and locknut to adjust the derailer's angle, something that most Simplex users don't bother to adjust to match their freewheel size.
I've even found that jacking up both spring tensions can cure certain cases of chain skipping and slipping on one or more cogs, where a used chain or freewheel isn't yet worn into a state of compatibility with the other.