hard to say if it was the same exactly. it probably was in the beginning, and later was at least based on it. thing is: the haptics are very close, and this is an, if not the most, important point of 55. it's not only about 'one' negative. it's about 'the' negative.
as panatomic-x was discontinued long ago, photographers used 55 even just for the neg. they were not interested in the pos or the instant at all.
it's the greys, the unbelievable sharpness (no way a 'neg' of a bleached fuji can reach that just a tiny bit. it's just a transparent paper-negative, not a real negative!), the unique way the material can solarize.
to save you some time clicking:
(at the bottom and around)
(at the bottom, by 'wirehead'
(at the buttom, 'bite the dust')
I'm putting useful comments like this into a post body so we can enable links - comments doesn't support that :(