It's not the 5th gearlocking pin, but an axle for a lever. I'll try to find a picture of it.

Anyhow, the remedy is to remove the gearbox mounts.
Lower the gearbox and engine (the engine mount stays put) about 10cm.
Open the end case of the gearbox.
Put parts to their correct places.
Refit everything.


Edit:
Ok, have a look at here:
http://www.kolumbus.fi/~w444776/JT_k...ox/rebuild.zip

picture 8243 specially.

edit2: