The code you are looking for is stamped into the case, not cast into it.
The code is situated on a machined surface up at the front end of the gearbox. The machined surface sits at 45o to the ground on the right hand side facing the right wheel. You'll need a wire brush.
Have a look at the Performance Ghia website, there is a engine and transmissions code page there.
http://www.performanceghia.com
MG