M Registers on early HP Calculators

[bshoring]

The Classic series, starting with the HP-35, has the scratch registers A & B, and then the XYZT stack represented internally by registers C-D-E-F, and then there is a M register.

On calculators that have a single storage register (HP-35, HP-80 and HP-21) it appears that the M register is the storage register. But what is the purpose of the M register in models that have more than one storage register?

Also, starting with the HP-21 and continuing through all the Woodstocks and Spice machines, there is an M1 and M2 register. Can someone enlighten me as to what's the purpose of those?

Does anyone know if the same structure is used on later models, like the Voyagers or later?

Thanks!