Pretty sure hp1/hp2 and hp3 were produced all at the same time between around 1989-1992. The hp3 were the drivers that were found to be less matched.
In 2003~ when Joe started offering his upgrade service he said that the stock hp-1/hp-2 drivers were identical and that if you were to send in a hp-1 he would dismantle the switches because it could only provide sonic interference.
I at one point owned several hp-1000 and found the hp-1 I had to sound the worst of the bunch, but could have been condition related.