I just tested the collet that came with my REX and the taper matches the one on my spare MTX3 flywheel.
Mugen does have a part # for a collet though (C0702B), but with Mugen's relationship with Novarossi i'm not suprised it has the same taper.
The taper on the collet that comes with the O.S. TR doesn't match the MTX3's flywheel.
I can say that most of the Nitro kits that I had built(RC10GT, XXXNT, 777, NTC3, NRS4) came with collets that specifically fit the kit's flywheel.
RACE WORLD HAWAII
Losi Type-R |Tamiya F104 PRO | Tamiya F103 | Mugen MTX-4R | AE FT GT2