Supermag, SuperMax, Maximum ???
I dunno.
Fatelvis
All I can tell you is I own and cherish a Monson, Mass DW Supermag and it is indeed chambered for and stamped 357 Maximum.
The Supermag Series just denoted the size of the frame, the Supermag being the largest of the frame sizes. The other two were small, chambered in 357 Mag and .22 or the large chambered in 41 and 44 Mag.
I handload all my ammo and use Remington 357 Maximum cases. Starline may still make the brass but I'm pretty sure only Remington is still offering it.
No one to my knowledge is manufacturing loaded ammo. You can still find some sitting on a shelf from time to time but that's about it. I really like Hornady's 180 gr XTP JHP with Hodgdon H110. Use a small rifle primer as indicated in the loading manuals.
If you want to fool around with some light bullets, go ahead, either keep the velocity down, or watch the forcing cone and keep a couple of spare barrels. In any case unless you shoot it a lot I don't think you'll have problems.
Take Care