This is Nibbsys pic of the head http://www.airgunspares.com/store/pr...t-No.-16-1026/ here are all of themhttp://www.airgunspares.com/store/search.asp
And here is the piston
http://www.airgunspares.com/store/pr...rt-No.-16-972/
As far as I know Ed the heads and pistons on the Mk 1 to 4 are all "keyhole" and the mk 5 is the circlip one.