How big are the holes in the piston skirt?
If they're big enough to put your finger through, then you've got a late model DT piston.
If they're itty-bitty little ones (about 10x10mm) then you've got a problem.
Of course, the real measurement is the centre of the gudgeon to the edge of the piston crown - bet that the piston you've got is 5mm taller than the TY piston (ie: late DT).
Early DTs take the same piston as TY, I think. I'll check tomorrow for you.
Wiseco do early DT pistons.
Failing that, get a short (100mm centre to centre) rod from a late DT from ProX.