This is the landing gear seal assembly from the lower end of a Jungmann gear strut. This particular assembly was modified by Mark Reninger and features (unusually) double O-rings and a lower seal retainer.
Here is the same thing, cleaned up and disassembled to show the seal, retainer and (for comparrison) an Honda fork seal as used in the assemblies modified by Woody Menear.
This is what a typical seal assembly looks like of the type favored by an Original CASA aircraft, a Joe Krybus conversion or a Woody Menear conversion.
The O-ring is typically an AN568-221 although it is not unusual to require a slightly smaller size. It is is tight fit and Vasoline is your friend when installing. My aircraft required a -219 O-ring. The exact seal dimensions/source depends on how/if the gear has been updated. These are some typical values:
For Joe Krybus gear legs the seals are 47mm OD x 35mm ID x 7mm thick. Part number S35x47x72MP low pressure seal from Metrics Seals inc Westfileld Indiana is a good fit. +1-317-896-3555
For Menear gear seals, a Honda motor cycle fork seal is used. The part number is 91255-292-003. These can be found on-line or through your local Honda dealer.
The gear legs modernized by Mark Reninger (who worked with Woody Menear for a while) use a thinner seal held in place by a plastic retainer and feature double O-rings.
The seal comes from McMaster-Carr and is their part number 9595K55 https://www.mcmaster.com/catalog/9505K55
A lesson I learned the hard way is that when installing the seal assembly on the strut, the threads at the top of the leg must be protected with suitable tape. Failing to do this will result in the treads damaging the inside surface of the seal and a leak is then pretty much guaranteed!