Hydraulic wheel damper for Steib S501

I have an original S501. It should have a hydraulic damper between the rear of the frame and the wheel swingarm, but it's missing. I am NOT talking about the two springs at the rear of the body. I have not been able to find out what modern replacement is suitable. No Steib parts suppliers in Germany (e.g., Ideal) or here in the US have any idea what to use. I can provide a clear labeled photo of the mountings on my chair if it would help.

I have seen a photo of an S501 fitted with a modern damper with a spring (basically a motorcycle rear shock)but don't know what it was.

Does anyone here know? Has anyone replaced the original damper? If so, with what?

Thanks!
Vintwin