Here's the answer from a very kind soul at Gables...
> The G1884 series started in 1965. I started in this business in
> 1967. I don't know the whole story but back then ACT control
> panels allowed all 8's to be selected. Something to do with a
> "remote keyer" function. Apparently this function was never
> adopted and later revisions to product and newer part numbers
> only allowed 0 thru 7 selections on each digit.
So there you have it! 8888 presumably put the code under remote control from some other piece of equipment.