I've been in touch with the seller on bay of fleas and his are custom made (assuming it is this one 285824109090 - if there is another I've missed when searching on there please can someone post the item number)

Keen to make simple changes - out of the box the HW80 has been so much better than the 99 and 95 that came before it, power has crept up a little from new, and it is almost 1980's long with the HW mod on the end of it, so there is a DM80 slimtech style 'barrel weight' on order, and while I have it apart to lop the end of the spring off having a thorough clean, deburr and decent grease makes sense.... and if there are fractions to be gained in smoothness by gliding it would be a good opportunity to do it all at once

Just not too sure about the lightening of the piston - shot cycle out of the box was smooth and docile, has become a little less docile as power has crept up (about .5ft/lb with 16gr pellet), see no reason for drastic machining if it's not required