The Sigma EF-500 seems to have been available in several sub-models … mine's an EF-500ST PA, bought for a song as "new old stock", knowing it was only fully compatible with my *
istDS.
I've used it on occasion as a basic "Pentax compatible" flash with some of my newer bodies, reverting to old fashioned Guide Numbers and appropriate manual settings on the camera as necessary, with some success.
For the odd occasion I want/need something more powerful or flexible than the on-camera flash I find it to be perfectly adequate, if a little less "slick and streamlined" than a fully P-TTL compatible unit … certainly a lot cheaper
Add a third-party Pentax-compatible hot-shoe extender cable and the combination results in a comprehensive flash solution that just needs a little more user input