I just picked up an older guitar today that has P90's in it. White soap bar. I took out the neck and it says Seymour Duncan and it says 1NC on the back. I didnt take out the bridge as it was a bit of a hassle getting this one back in.
My guess would be Custom Hot, That's what I have in my Hamer Special, although I haven't seen the back of the pups to confirm this. Though they are hot as sh1t. Maybe somone will come along with a more definite answer.