It's actually a 1855-0566, no mistake. The only thing I can figure is maybe it was a proprietary part specifically ordered by HP for this instrument.
HP6944A Series 200 Multiprogrammer.
The manual just lists it as a FET made by, you guessed it, Hewlett Packard.
I'm probably SOL, unless I find me a scrap unit to salvage the part from.
Get this, for anybody interested in trivia on this instrument:
The instrument also has a blown VSK3040T, which is a 30A, center-tapped dual schottky rectifier.
So far, the only company that my company will allow me to buy it from, wants $1800 for one each made by Varo,.
... I'd like to wring the neck of whoever the engineer was that designed the power supplies in this thing.
Thanks everybody for trying to cross this FET for me.
The owner wants it fixed, so my next move is to buy some scrap units and salvage parts out of them.