question Bedford piston markings

[No Signals]

Got the head off to do a 'once over' and see that as well as the 'Y' tolerance mark the piston tops have a 'X' as well. This is not orientated any particular direction, i.e. it doesn't appear to be designating 'front' / whatever. Doesnt follow any obvious layout pattern from one piston to the next. No reference to X's in any of my manuals. What is it indicating? Cheers.