Yes, that's the issue. The stocker fits the high top carb but the inner length isn't quite enough for the larger slide and it's housing, although the cap is lower. Strangely the cable I have does not appear to be a stock M model cable. As far as I can tell, the 76/77 cable appears to be what I need. The MotionPro one is claimed to fit 76/77/78/79 varieties of PE/RM with the larger carbies, and *looks* like what I need, and at about $20 landed shouldn't break the bank if it's not right. Luckily I have several spare throttle/twistgrip assemblies so reckon one will fit.
We shall see.