I believe the HP1s are planar magnetic phones,I have neither seen or heard your particular model but do have a pair of
They are not electrostatic but do have in common the fact that
the diaphragm is driven over its whole surface.
Such transducers are used in speakers such as Magnepans,and
detailed descriptions of their working principles can be found with
The attached picture is of a disassembled transducer from
a pair of Wharfdale Headphones[from about 1976] using
the planar principle.
The voice coil is 'printed' on the copper coloured diaphragm
in the black frame.
The perforated panel is a magnet ,and the diaphragm is enclosed
between two of these.
As for value ,My HP2s cost me £40.
I doubt that the HP1s would be worth much in cash terms ,but they are a rare and interesting bit of headphone history so treasure them.
Also I believe their are a couple of other people here who own a
pair,, Beagle springs too mind.
OH how about posting a pic of them?