SMK were able to supply mine through my RFD.
The part is BT6M22AYL
The dimensions are tube 18mm rod 8mm loa 260mm. rod is 110 long (protruding out) and tube 149 long.
There is a gas ram company in Derby that can make them but they would need one to compare.
A refill is £18. You can put gas in but you cant take it out.