LT77 gearbox identification

My lightweight has an lt77 box and 1.192 transfer box and as a result is quite high geared which can be a problem

Trying to work out which transer box would be best using ashcrofts ratio calculator so i wrote down my gearbox serial number which is 31A0276578E follwed below (stamped in bigger numbers) 5180

I cant fine reference to a 31A box anywhere does anyone have any ideas ?

Thanks

Ask Ashcroft.