could anyone shed any light on the origins of this gearbox OR IF IVE GOT THE RIGHT NUMBER, all part of my endeavours to get the truck through the IVA this is the only number i could find after a bit of careful paint removal 56A0017833G pic attached so you can see where the I found the number, OK i have found a web page that suggests it may be a disco gearbox , just need clarification if anyone can help

According to the Ashcroft site, 56A is a "long stick" (defender) LT77. Suffix G is one of the later types.
From their site:

LT77 Development

The LT77 was introduced to the Landrover range in approx 1983 with the suffix 'D' box and was developed over the years to the suffix 'H' which was last used in 1994.

The 'D' and 'E' are very similar the only difference being the reverse gear was a little fatter in the E,

When they went from 'E' to 'F' larger layshaft bearings were used.

The 'G' was further improved by all the gear teeth including the pinion getting approx 20% thicker. Also a much wider pump was used to increase the lubrication and cope with the fact that some of the LT77's were now being supplied oil cooled, ie V8 NAS 90, 3.9 RR Classic and the 3.5 EFI Discos.

The 'H' (termed the LT 77S) was the same as the 'G' in terms of strength but used a different synchro set up called the'Double Synchro', In reality it didn't work much better.

Upgrades to the R 380

Should the LT77 fail on your vehicle you will usually have the opportunity to replace it with a later R 380, unfortunately surcharges will apply but if you are either planning to keep the vehicle, working it hard for a living or going on an overland trip where both reliability and the facility to add an oil cooler are important then this additional expenditure may well be worthwhile.

56a has 1 tooth extra on input pinion the same as rrc ordisco boxes 22 instead of 50a or 51a 21 ,fitted to 200tdi defender whilst 50s fitted all other 4 cylinders, 51a been military, though they fit and look exactly the same as each other