Worked a deal out with the seller - we are both happy and they learned about removing the light for shipment.
GreyQ - the "silver" is the nickel stitch length circle and the Singer emblem underneath.
Here is a "regular" 15 along side the "silver" 15. Yes, I know only one is a 15-91, but the 15-90 was handier to grab.
And the detail - I love that the stitch length shows in the little window.
What you commonly see for the 15-88, 89, 90 and 91
From what I've seen, I think this was how Singer made the first 15-91's. Some sellers say "only 5,000 made!" but that is only for that run. Just from what was on eBay now, I've found machine from three different runs, one in 1931 and two in 1932. I guess that about 40,000 were made this way - so still quite rare, but no way only 5,000.