The clue is - the stamped 'front' marking.
Split to front (non-thrust face), thrust face is rear face...for a 'forward rotating' (which way does it rotate when the machine goes 'forward'?) engine....
Some numpty has put the piston in the wrong way round....either that or they've made some radical alterations to make the engine rotate in the opposite direction
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)