There is some stuff about Adjusting bottom to Rear joint, spring washers in Strida 1 manual:
issuu.com/mark77a/docs/strida1-owners-manual/44
&
issuu.com/mark77a/docs/strida1-owners-manual/19
Looks like the 2 moving parts are effectively spring-loaded together - to get enough friction to prevent bottom tube falling to the ground, and enough torque transfer between bottom tube and rear joint
I’ve found that having a large diameter nylon washer between the bottom and rear joint flat surfaces helps get good both friction and torque transfer. And as its hard to find various spring washer thickness’, as in the manual, varying the thickness of this nylon washer also adjusts the axial spring load on the joint.