Aussie 4speed m20 stuck in reverse

I have just bought an aussie m20 4 speed gear box to use in my HG van project. The gearbox is on the bench and is in reverse gear, as shown when I wind the input shaft. I have tried to move the flat plate lever that goes onto the selector shaft in the left top of the extension housing; it wont move. I have taken the plate off, and used a 3/8 open ender to try to move the selector shaft , but it refuses to move out of reverse gear.
Should I remove the extension housing?
If I remove the extension housing, will there be any difficulty refitting it?
Any suggestions?

Make 100% sure all forward gear selectors are in neutral or you cannot pull it from reverse.

Regards, Smooth

sinbin wrote:Make 100% sure all forward gear selectors are in neutral or you cannot pull it from reverse.

Regards, Smooth

It is definately in reverse only Smooth, as I can rotate the input shaft, and the outout shaft rotates in the opposite direction.

I notice that, when the nut is done up tight, the lever plate on the reverse selector shaft binds on the surface of the extension housing.

I have taken the side cover off to look inside the box, and can see all synchro rings, so again, no foeward gear is engaged.

Its got me stuffed, but then my last look inside a gear box was around 1967 when I regularly dismantled my XL falcon box. This m20, while also being a bit ancient, has me hesitant to remove the extension housing as I presume reverse gears are in there, and I fear mystery parts flying out if I open it up.