Back on the thread, & further to my last post, I have never seen a Mk5 M86 250 pursang barrel, but it may be diff to M103 barrel. I suspect it maybe like the Mk5 M107 Matador barrel which had the narrower transfers (& 's' shaped finger ports cut in the liner). If so, it should go straight on the M100 bottom end w/out need to open up the transfers in the cases(as would an M107 barrel), but the porting wouldn't be as good as M103 w 4 proper transfers, I wouldn't think.
However, there are a lot more M103 or frontera barrels around than M86 barrels. Actually, the Frontera barrels (M152) have more radical porting than the M103, but technically aren't legal for pre75, tho the 103 barrels can be brought up to 152 specs quite easily.
My friend mentioned above also has an M120 Mk7 250 w legal barrel (M103) mildly ported & Pomeroy pipe from Hugh's in US & it absolutely explodes out of corners like a scaled cat - just like I remember Gary Flood doing back in the day.
Anyway, enough of my rambling. Grab it, & happy pursang-ing! Properly set up, they're a real sweet race bike. Certainly one of my very favourites. Tho remember the Mk6 is still RH gearshift (but can be converted to LH).