It is not from HP but might be from interest for you, too.
Texas Instruments is in the process of moving the Educational Department and the employees started some house cleaning. One employee discovered a never released prototype - and it arrived less than 10 minutes ago here at the Datamath Calculator Museum.
It is a real triple-P:
One P for Programmable
One P for Printing
One P for Prototype
I would guess that it was developed about 20 years ago, later than e.g. the HP-41C.
It will be featured in December in the Museum - today just a small teaser:
Have a great weekend.
Regards,
Joerg