My recollection is the M59/66 is about .001" OD bigger than the others on the piston head and also ever so slightly shorter(like 1/32") due to the gas valve. IIRC the guy who was making them and selling on Ebay for a while back in the day determined that and made sure people made the distinction as to which they had before purchase so he could ship out the correct one.
Also, are you missing one or are you blowing gas out the tube? If the latter you may have a gas valve issue.
On another note, are you missing the receiver cover? I have an M59/66 cover if you need one.