I picked up a ratio adapter at the swap meet last week end.
It appears to be an older one made by Stewart Warner. It has a marking on it of 1.2020. I don't know if is anything that would help your situation or not. It would be good to know what the code is in relationship to MPH corrections.
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