Series 2 intermediate gear help!

We have an old series 2a (i think) 1964. we need to replace the inter gear, shaft. bearings etc. The parts supplier needs to know the suffix of the box, a b c or d. We have no identification numbers to work with. They say that there should be a mark on the inter shaft, there is not. Can anyone help me to identify the suffix? Should there be any numbers on the t box or gear box? If so whereabouts?
Thanks

the gearbox number should be stamped into the side of the 'box on the nearside i believe

I will clean it off and have a good look. Cheers