I have this piston head http://www.ak2m4.co.uk/internal-parts/pistons-heads/shs-piston-head-aluminum
This cylinder head http://www.ak2m4.co.uk/internal-parts/cylinders-heads/shs-m4-cylinder-head-pad (although the pad has been replaced with an Airlab sorbo pad).
I was mistaken about saying my piston head has 2 o-rings - I was thinking of the cylinder head. Even so, I would suggest that maybe the quality of the seal itself could be better (mine has an Airlab one in it).