It was an after-thought, caused by player pressure and booklets by Lenny Fromm in the early 80s about the payback on 9/6 JOB. Think it was quicker and cheaper to attach the plaque and chips than buy and install new glass. IGT at that time probably charged $75 for the glass vs about $1 for the plaque. Think I have chips that read "2 Pair ". I can recall seeing players at a two pair type of payout and 6 feet away there was a 9/6 machine very lonely wondering why nobody was playing. Much later, the Gold Coast set up a 9/6 JOB $5 carousel with a progressive that lasted many years until ownership ordered 9/5 after a one or two month lower profit. Players went elsewhere and good friend Bob Geddes , after a two months, came up with a new pay schedule for the casino that helped but the sharp players played at other casinos with full 9/6 progressive. There were always enough players who played less than perfect strategy to make those progressives profitable. One wealthy lady would play when the progressive was positive [ $23,000 ], but she only went for the royal on most hands. Bob calculated her loss rate at something like 6 or 8K per hr, if I remember.
On a somewhat related subject Ceronix stopped fixing the monitor chassis about 4 months ago for Fort 1s , 2s, PEs etc The chassis is Model 1492 and luckily I have a few good ones available.