Heikki!
'Crashbox' is slang for a non syncromesh transmission. You already know about 'double clutching', which is the method used to provide smooth shifting of those transmissions. Some folks who were not very adept at that procedure would try to 'force' the gears and consequently created a great 'clashing' and 'grinding' of gears; or, 'crashing'. Hence; 'crashbox'. (lots of people still try to shift that way!!)
Regards, George
