In message , Terry Harper
writes

50 or 60 years ago, almost all cars had crash gears on bottom, and
Rovers had it on first and second gears, but they also had a freewheel,
as I recall. The Bedford OB, for example, had a crash gearbox, while
the Dennis Lancet III had a crash box on 1 to 4, but a pre-selector
overdrive 5th speed. Some were "constant mesh" gearboxes, where dog
clutches engaged the gears.
I understood all crash boxes to be constant mesh.
--
Clive