Have discovered something whilst rebuilding my gearbox - thought I'd share it. It appears there is some difference is size between the standard F5C71B internals and those that were prepared for competition. If you therefore have a competition box that needs new cogs, etc you will struggle to get replacement parts. Chatting with my local Nissan dealership, it transpires that there are no seperate part numbers to differentiate between the internals of a standard box and a competition one. I scoured through some of the period sales literature and the part numbers listed are those for the standard box.
I have identified three types of gearbox -
the standard one, factory special - with thicker laygear, drive gears but standard reverse and idler, and the good old straight cut dog box. Interested if others have anything to add on this.