What is the difference if any between a tjet arm and a tjet motor modeling arm ?

About 36 ohms. The Model Motoring arms are notorious for ohming in the 50s. they would usually get about half way around the track and over heat and melt. I have some that have done that very thing and some that run like a top, but they're definitely hit and miss.
hojoe

thank you just wondering

Well, let's be a little clearer on this. The Model Motoring armatures were supposed to be 15 Ohms, but the factory in China (possibly due to the language barrier) produced 50 Ohm armatures. These were a mistake, not manufactured this way by design.

There was a time when MM offered replacement 15 Ohm arms if you received a 50 Ohm arm. I got a few myself.

I do not know how long the supply of 50 Ohm armatures lasted. So, if you see a MM armature, you will not know whether it is 15 or 50 unless you measure it.

Joe

The MM arms are triple-lam, where the Auroras are double-lam. Also, MM arms only used gold-copper wire.