Many years ago Schnitzer made rear dampers for a Series IIIA we used on LeJog, they took ages, but were of good quality. I doubt though that they are still going. Other than that I have heard this company have done work on a Series Rapier race car http://www.corneringforce.com/ Sorry can't help more. G
Bit of a long shot this one!! Working on a 1961 Rapier race car and the rear shocks are rubbish. Need to replace them, but also need to be careful what we fit. Only allowed twin tube steel body type with adjustable bump and rebound. Have contacted 'GAZ' who do quite a lot for us, but just wondered if anyone had seen any other manufacturers with any.