What is the brand that Tom Dunson (John Wayne) draws at the end of Red River (1948)?

The brand that Tom Dunson (John Wayne) draws at the end of Red River (1948) was a river with a D on the top and an M on the bottom.

The D is for Dunson, the M is for Matthew Garth (Montgomery Clift).

Dunson tells Garth, “You’ve earned it.”