The MX is a fully mechanical SLR that was produced from 1976-1984. Noted for being considerably smaller than its predecessors and offering only manual controls, it was Pentax's "professional" offering released alongside the electronic and fully automatic ME.
For me, this camera does a lot right - it feels solid yet compact, it's fully mechanical, includes a self-timer/DOF preview, plus offered an array of interchangeable focus screens and specialty backs. And the little aperture window in the viewfinder makes me wonder why more cameras didn't include this optically fun and very handy feature.
Not without its drawbacks - the lack of exposure compensation and notoriously stiff shutter speed dial can be annoying to some.
By 1984 when production of the MX ended, Pentax was already in production of their next top-level SLR which included nearly every pro feature you could dream of. Alphabetically backwards, they called it the LX.