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.