Just a Guess, as I don't have my reference material in front of me, but...
1979 to 1985 rang on the date. I'd guess '82 to '85
And it sounds like it's some variant on the PH10 or UO9/10/14. I'm not sure the P8 ever came with Simplex derailleurs.
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I have a white PX-10, a Green Dawes Galaxy and an Orange Falcon, now I'm done.