It's a 1931 Ford Model 68B Convertible built as a Montana mail carrier.
It is one of two custom-built in Glendive, Montana in 1941 in order to navigate through snow-drifts!
It has Goodyear diamond tread tractor tyres and a Model TT truck differential low-geared to allow for the over-sized tyres.
The US Mail took getting mail delivered in Montana very seriously!