From the image shown, is the rear bearing holder brazed onto the piston ?
It doesn't look like an LGV piston, but it does look very much like an early Venom altered one.
Another area where the compression tube can leak which hasn't been mentioned, is around the end plug which apparently is screwed into the tube.
From memory the test for this is to push the piston into the comp tube with the transfer port blocked while the whole lot is submerged under water --- then look for bubbles coming from the joint.
All the best Mick