Are Mondasian Cybermen simply the original cybermen, or are they a different race?
The original cybermen were the mondasian. New who muddied it with the alternate earth cybus men whom then took up residence on our universe and have been the only ones used since in new who.
The audio Spare Parts is still Imo the best genesis of mondasian cybermen.
It's all a bit weird now, but the way I see this now is this:
1) Cybermen develop naturally on Mondas as the Mondasian people struggle to survive. Mondas is basically a twin planet to Earth with a very similar ecosystem and its indigenous race more or less identical to humans. (Big Finish story Spare Parts)
2) At some point prior to this, a ship departs Mondas /before/ the cyber stuff reaches its natural conclusion in parallel to that on Mondas itself - and we joined that ship this week. It's possible The Master used his knowledge of natural events on Mondas to 'help' the Mondasian colony ship along, too.
3) The events on Mondas are mirrored on a parallel Earth, but with the reasoning for their creation less desperation and more evil/maniacal (and Davros-like), resulting in a more militant style of Cybermen with the same basic setup. (Rise of the Cybermen) There are key differences though, like Parallel Cybermen being a human brain in an 'immortal' metal exoskeleton (again, fairly Dalek-like), where Mondasian ones are more whole humans encased in metal.
4) At some point in the far future the two species of Cybermen encounter each other, ally, and eventually merge before waging war across the galaxy. (Nightmare in Silver). We know they're merged not only because of author comments but also because the Cybermen in that episode resemble the Lumic/Parallel Earth ones but also have traits and skills associated with the Mondas Cybermen including a weakness to gold and (as seen in Death in Heaven, which features the same style of Cybermen) entire bodies being encased in their armor rather than just brains.