Yes MarkT, the puller that just arrived today is a hollow bolt without the inner bolt...but it isn't a fit either @ 20mm x 1.5mm. It won't thread more than 1/8 turn and STOPS. I sure would like to find a traditional 'inner / outer' bolt style puller.
The 19mm x 1.0mm I have turns in a bit more than I said in an earlier post. It threads in 1 whole turn and stops solid.
Also I can just get the vernier caliper in between the crank end and the female threads to which it measure a solid 18.88mm. So it looks like we have a 19mm diameter but not a 1.0mm pitch.
Visually it looks to be a 1.5mm pitch. Could be a 1.25mm. I don't have a 1.25mm 'bolt' to visually compare it to. But the mag threads look very similar to the 1.5mm pitch I have with this new 20mm puller.
I'm a bit scared to use a 'jaw' puller on the back side of the mag. It's aluminum.
Thanks guys, I'm open to suggestions....
The search continues for a puller....
 

The puller RT325 posted looks to be this exact item from Bikemaster. It states a 20mm x 1.5mm thread size on their website.

bikemaster.com/bikemaster-magneto-flywhe...-offroad-150604.html

I've tried a...
19mm x 1.0mm
20mm x 1.5mm
of which don't match up.
One item I didn't mention is the condition of the threads on this MX 360. They look great so I don't think corrosion is an issue with thread binding.
On the other hand, the DT3 mags I pulled with the traditional 'inner / outer' bolt puller had major corrosion / rust and still I could tell they were the correct thread size. I used anti-seize and patience on the threads and they ended up threading on to full depth.
 

 Ok, the genuine puller i thought i had turned out to be probably yz80 & is smaller. But wait there's more lol. I have a multi use puller & 20mm is it in rotor off an early 250 or something. Ignore the stator as that's a bit later. So 20mm screws in perfect. I don't have thread gauge but randomly sitting an 11mm x 1.5 tap it lines up so does that mean it's 1.5? even though its 20mm--i'm thinking it does?. Pics coming.Ok, the genuine puller i thought i had turned out to be probably yz80 & is smaller. But wait there's more lol. I have a multi use puller & 20mm is it in rotor off an early 250 or something. Ignore the stator as that's a bit later. So 20mm screws in perfect. I don't have thread gauge but randomly sitting an 11mm x 1.5 tap it lines up so does that mean it's 1.5? even though its 20mm--i'm thinking it does?. Pics coming.

Just went & rechecked that tap as looks not perfect but yes it fits perfect if i line it up properly & hold it up to the light.

RT325, thank you for those pictures.  What's missing here is we have no idea what puller is being used...  if the "hollow" is not deep enough to get a few threads started that could be the whole issue? 

Really need to have the right one, not just the correct thread.

Yeah the puller i pictured is hollow, but 'way too hollow' so would need something dropped down in the puller long or short enough to get a good bite for the puller but still a turn or two left for it to--pull. But easy to do. So proper original with the bike original kit would look like one of the hollow ones on the bench in the pic but 20mm version & possibly a bit deeper. I had one--must check my riding boots as happened before & things fall in off the wall lol, including the damn speedo cable retaining clip i found one day, after i'd given up, stuck pointy end up in the sole of my work boots & walking around with it.

Wow...great information guys.
Ok here are the pullers I am working with so far...
The 20mm turns in 1/8 turn and stops solid.
 

That's the same rotor as mine so in theory that 20mm motion pro puller should work providing its hollow way down inside. I'm out of ideas sorry but i'd steer clear of a 3 jaw puller if i could. Can you hold the rotor without damage & force the puller a little--shouldn't have to i know. Stopping solid as you say, does sound more like its reached the end inside the puller rather than wrong thread pitch which would tighten to a halt but not stop dead.

What does the other end of the Motion Pro look like?  How deep is the recess in the end?

I found my old inner rotor puller...  might be OEM, I've had it forever...  threads right into an old Yamaha inner rotor I have  that someone ruined by trying to use a jaw puller to remove. 

ID of threads in rotor are about 18.6mm.

OD of puller threads is just under 20mm...  19.86 or so.  Pitch is absolutely 1.5

Recess in end of puller bolt is 19mm deep and 13.6mm diameter to fit over end of crank threads far enough to grab a few threads in rotor before bottom of recess hits end of crank.

Measurements made with cheap Harbor Freight caliper.

I looked up Motion Pro and they have two 20 x 1.5 pullers.  One is flat on end and one has a recess.  No dimensions on the recess but they list it for all kinds of Honda four stroke models and list a Honda part number.  Nothing about Yamaha.  Could be the right thread but the wrong recess dimensions?