Hello Adulthuman,
I’m afraid it will be impossible to find this part, please consider that the last bikes of these Strida version were manufactured ~ 23 years ago!
Since 2014 are even parts for the following version - the Strida 3 - not any longer in production.
All you can do is:
- Try to find another (defective) MK1 with a functional socket part.
- Try to contact the inventor, Mr. Mark Sanders directly, a while ago he still had (some) parts, perhaps he can help.
http://www.mas-design.com/
Good luck!