This limited edition Citadel miniature was a 1985 sculpt and went by “Trooper in Power Armour” according to the blister.
He has “PA TROOPER” on the front of the tab and GW 1985 on the back. The two letters on his upper backpack are P and A as well.
I’m confident it was sculpted by Bob Naismith but so far I haven’t found a catalogue entry to confirm that.
The modern Thallax Cohort miniatures were apparently based on this guy’s sculpt.