This MIGHT be from 1983, but I doubt it. MINE was from 1983 and mine is designed way different than yours. But mine might be one of the last ITT versions before switching back to AE. So it's a possibility this is from 1983.
The ballast seems to be mounted similar to one of the newer 115's. With the bracket going OVER the balast and using pressure to mount in the ballast. The 115's made today are like this, my new 115 had the ballast mounted this way.
The "pressure bracket" mouting is to allow easier ballast replacement (not that linemen typically replace ballasts). It's easier to loosen 2 screws than it is to remove 4.
The B2255 also had used the bracket to clamp down on the ballast...only thing is the bracket was attached with four screws. But you could mount the ballast on the door so you could change them out easier.
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The design looks like an AE though, cause the slipfitter has one plate instead of two clamp plates like mine has.
It's funny all of these companies had different brands going everywhere except GE and Hubbel.