Stuart;
Welcome to this Forum!
I've seen this before...it's not a huge problem, but what you have experienced on your Damper is a result of either a backfire and/or possibly too thick a Damping oil...or a combination of the two...
As the Dashpot tried to quickly rise (against the CLOSED Damping valve), oil could not bypass the valve-assembly quick enough so the force transferred to Valve threaded top was too much for it to take, (SOMETHING HAD TO GIVE!) and it blew the rod right through the plastic cap...
See also:
http://www.sw-em.com/su_carbs.htm#Da...lve%20Assembly
I recommend you check your Damping oil viscosity (should be 20W, not thicker)...also don't stomp on the accelerator pedal before engine is warmed up, resulting in a lean backfire. Also check your Choke is enriching as it should! You MIGHT be able to (super)clean the area, file some burs into the rod to give it some "tooth", push it back into its normal position, and epoxy it into place...otherwise, replacement is always an option, but check other things I mention or it might happen again with a replacement...!
Cheers from Connecticut!