weird xs2 engine

[CHAZZ]

hi guys i just got hold of a rough xs2,so did the usual and took all the bolt on crap of it,the bike had bad compression so engine strip,well here,s were it gets weird,markings on the base of left cylinder reads 001 the right pot has 003. left piston has on the crown 949 and the right piston has 951, right piston is very tight in the bores left piston is quite free.ive owned a couple of xs2s and tx650s and never come across this before,any ideas

 

[BowlingSS]

I am not sure. Someone here should know. I have seen the markings on the cylinder sleeves before.