That's called a flange, wheel flange, axle flange, brake drum flange. Depending who your talking to.
The distance between the brake backing plate is somewhat meaningless. IF the unit has wider brake shoes and Thus a wider brake drum Then the axle tube extends deeper into the backing plate so the drum does not protrude into the wheels rim. The difference in 'width' or 'distance' between setups can be an inch or more.
Far as answering yyou question of flange to flange measurement ..... I can't. At one time I had it measured to the gnats gonad but damned if I can remember now. I should have written it down.