Hoya actually made SLR lenses in different mounts, including entax K, Nikkor F and Minolta MD and etc. Some examples:-
(Source: http://www.flickr.com/photos/garydanton/5769178099/)
(Source: http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&item=270310838388&nma=true&rt=nc&si=WJoBecR6IQtD6asTCB%252FpIvX7mVg%253D)
Well, the printed HMC name as seen is not on the filter, but really on the lens itself instead! :-o
Anyway, Hoya is just a name of the past in the history of Pentax, as long as the production of cameras and lenses is concerned.
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Hotax Has NOW Been Rictax! ;-D