Originally posted by Lord Lucan The Pentax MX (and to some extent the ME) was a direct response to the Olympus OM-1.
I don't think that had anything to do with the LX which was a K2 DMD replacement and meant to rival Nikon and Canon in the "heavyweight" professional market, even though it was less bulky than its rivals. I think Pentax were gambling somewhat on a trend to smaller cameras in all parts of the market. Looking at the Canon F1-N of 1982, that did not happen.
The LX took no design queues from the K2 as the latter used a vertical shutter, did not have interchangeable screens or backs, motor drive only with the K2 DMD and viewfinder metering. And unlike the LX, OM2&4, it cannot meter very long aperture priority shots.
Like the MX is to the OM-1, the Pentax LX also borrowed a few technical points from the OM-2 and greatly enhanced them such as off the film meter, TTL flash gigantic viewfinder and of course physical size - with full interchangeable viewfinder!
As far as I can tell, the OM2 was the first production implementation of off the film metering although prior patents indicate Canon and Minolta experimented with it. The OM2 implementation required two metering methods with a cel in the mirror box for auto mode and cels in the viewfinder for manual mode while the LX used a lone cel in the mirror box for both. Later on Olympus used only one cel in the mirror box for all metering in their OM3&4 models.
The LX uses horizontal shutters like the whole OM single digit series except that it is a titanium foil.
The OM2&4 can reliably aperture priority autoexpose for minutes while the LX greatly extended this capability by being the only camera ever - past or present, that can do so for as long as it takes.