I and others have found that Pentax SLR's with the horizontal travelling cloth shutter can "pre-release" the mirror (aka mirror lock up) by flicking the shutter or tapping the shutter release lightly. It's described within some other threads or elsewhere on the web. It takes just a bit of practice and isn't difficult to do reliably. I've a Spotmatic and a MX in which the mirror can be pre-released in this way.
old Pentax 35mm film camera without mirror lockup
in Pentax
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