why would it be a disaster to prime using acrylic or SBR if its a gypsum plaster? that's what i would normally expect to be used e.g BAL APD, tilemater Prime plus etc etc....
There's not much info on Seraphite, or Seraphinite plaster on tinterweb. I have heard of it but not sure if I've ever seen it. Given many of us have worked or been in 50s and 60s houses I guess we've probably all seen it but not known.
I think it is a fore runner to Carlite browning type plasters which were used between when lime plasters with horse hair in were the thing and, modern wall boards, and before high shine gypsum finish plaster became a thing. I'm fairly sure is calcium silicate based but couldn't swear to it...