Due to not much compression on the CT2 I recently acquired I pulled the jug off to see what I had.
It has a piston with ART on the inside. Piston measures about 66mm at the skirt which would indicate
to me it is a standard piston. It is stamped with 97 6 on the top and that has me confused. I measured
this with my callipers and I don't have a bore micrometer to check the bore. Anyone have any experience 
with these pistons?

ART is original Yamaha.

That piston is the standard size original piston . It's size would be 65.976 mm , which is what the numbers refer to .
A first oversize OEM would be stamped .25 , being for a 66.25 mm bore .

If original piston those marks indicate a size of 65.976 mm.

Yamaha stock bore target was 66.000 mm...  you might still find an ink stamp on the cylinder skirt that slides into the cases...  that would have indicated how far bore was from 66.000...  on the assembly line they could then pick a piston to get the bore clearance correct. 

Yes, ART was an OEM for Yamaha...  they were also sold aftermarket...  most famously Pro-X sometimes supplies ART pistons.

Standard after 50 years is pretty good but if it's worn the rings out--low comp--it might be time to bore it rather than just a standard kit. But best ask your machinist if he thinks it'll clean up at 10 thou over 0.25mm as doesn't take much wear at the port area to need to go straight to second over. Was just a thought.