Quote Originally Posted by BTDT View Post
The HW50 piston weighs 186 gm, the HW55 163 gm. The HW55 piston head is aluminium, and extends back into the piston, giving available length for the compressed mainspring of 95.6mm in the 55, against 108mm in the 50.

I suspect that if Venom used the HW50 piston in their 55, it was as much for the extra spring room as the mass.
I really need to open up a few of the ones we have in the family, we have to different 55's and several different 50's, would be interesting to see if there are even more variations between them.