GEC (Parmar/Parry's) 400w SON Ballast BNOS

Brand new old stock, a big heavy lump this. I think these were recently/are still made under the Venture brand, they look identical anyway.

There's loads of these still in use at my work in the one bay that still has 400w SON high-bays.

This has '89 D' printed on it at the other and, could be a date code?