I have one of the aforementioned speedometer ratio adapters but I have no idea what ratios it changes. It is made by Stewart-Warner with the following numbers: 668F S12 1 2020 X. If anyone knows how to decode these numbers it may be usable to somebody.
I have no use for it so if someone is interested you can PM me.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine