When I say "matched set" I don't really mean that the flywheel can never be swapped...  just that if it is swapped you need to use the part number that goes with the stator part number.   Over the years I've seen several people try flywheels that "fit" just fine but are from a different style stator and won't make spark at all with their stator... 

Kind of like a Kwikset brand key and lock...  any key made for that lock opens it just fine...  but another Kwikset key could fit just fine into the lock but it won't open it because it was made for a different lock.  Different flywheel designs can have magnets placed differently so even though they "fit", the voltage spike isn't timed to happen at or near the time the points open = no spark.

And of course even if a matching flywheel to that stator is used, it has to be good...  it could have been dropped or heated or in some way lost its magnetism. 

Wow this is interesting stuff & thanks for posting.
I do wonder how much thickness off alloy is sitting over the magnets so hope she's safe. I have a similarly machined one i use sometimes on my RT2 & its still hanging in there.
But to tell a story i see the rivets have been hammered at some stage of its life.
I 'had' to do that to mine during a roadrace meeting--piston port RT1--as the flywheel came loose on the rivets & gave an unusual abnormal vibration.
So when i found what it was it was off with the flywheel & my wife holding it on the trailer drawbar & me hammering with something, probably a socket extension & hammer.
Out for the next race going smoothly then it stopped & all the rivets had broken.
Was running without a mag cover to help cooling & the flywheel 'flew' & took off on its own. It went for a long way. All vibration stopped at that point haha.
Talking rusty cam--one of mine is pitted but still working.
I just spun the flywheel in the lathe & cleaned it up best as could with emery then wet & dry paper with oil on it.
My theory after that was the smooth pits hold some grease so was all ok.

Mark, makes sense. Forgot to consider that the used, almost 50 year old part could have bad haha.

RT, hard to say how much material is left holding the magnets, Ill have to watch out for any weird vibrations then and keep an eye on the rivets good thing to keep in mind. And the pits ARE going to be grease pockets haha nice theory! that makes me feel better.

Measured my machines rt2 type flywheel & is 131.2mm
I had to google to convert yours to mm & looks like i'm about one mm safer.
Didn't have to step mine down near the back for clearance though.
I'll sleep tonight haha.