Not an accessory. This plate was the final authority as to where and
when your car was manufactured.
Lots of accessories available in 1931. Also some
good after-market stuff like the Harrison hot water heater. The
mileage counter was a dealer-installed item used as a friendly reminder
when to get your car serviced next.
(Click on pictures to enlarge them.)
Harrison Heater (after market accessory)
Two Harrisons on left, and
the smaller Chev heater on the right