Yeh, I've always found that gap weird too. As I recall some of the micro-mettises didn't have it - & look better to my mind as a result.
MX, I'm pretty sure that tap is OEM. Look like typical/common british taps of the era. Look exactly the same as on Dad's Villiers pump engines! They were horrid. Relied on cork cylinder inside to seal, which it usually didn't for long. Cork swelled, jammed etc. You get the idea
Regards, weight. In Oct 72 Dirt Bike rolled all the common dirtbikes straight out of the showroom floor & weighed them on a calibrated scale then published them. They didn't say if any had any fuel in the tanks, but the 72 Rickman Montesa weighed 228lbs. That surprises me cos Monty frames are heavy (My V75 frame & swingarm weighs 20kg) & one would think a specialty frame would be lighter.
VR engine maybe slightly lighter than 72 63M engine, but probably only by a kilo or so.