I can now answer my own question. The box is 1959.
I’m glad I opened it up to fit new springs, as I also needed to fit a new lay shaft bush as the old one was broken, naturally as well as springs I also renewed the bearings.
And…………. It also had 3 gears with at least one tooth missing, with the remnants inside the box as well as a random nut (how?)
I looked up the gears part numbers and managed to get some really good s/h ones off eBay. Reassembled and it’s put away until time to use it.
Of course, once I’d done all of that I looked through the boxes of “other bits” that came with the bike and found 3/4 of a 1960 gearbox that had all 3 gears in good condition. 
I could have used them!
I also found a very good set of TR6 crankcases, and a reasonable spare crank (al[ATTACH=full]115873[/ATTACH][ATTACH=full]115874[/ATTACH][ATTACH=full]115875[/ATTACH]though that has gone away to be properly assessed to see if it’s any good.
And the cherry on top of the cake is a pair of mk11 Morgo barrels and 0.00 pistons in nearly new condition.
