The wading plug dose not go into the gearbox, but into the bell housing between the gearbox and engine and contains the clutch or torqueconverter depending on gearbox type.
I have no experiance of R380 gearboxs, but unless you are confusing bell housing with gearbox then there is not wading plug on the actual gearbox of either an LT77 5 speed manual or the ZF 4 speed auto.
On my V8 there is a dummy hole in the bell housing (about 4 inchs from the actuall hole) which contains the wading plug for storage between use. If you don't have a plug then they only cost a couple of pounds to buy new and they should be better than having a bolt hanging down.
