model a gearbox, help wanted!!

just need a little info if you can help? i have 2 model a gearboxes and both have damaged casings at the rear and have spat the snap ring out, are there different boxes for diffrent years?? and are there different sizes of case? just wondering if ive broken them when fitting or if its a coincidence there both bust. is this a common problem and what causes it?? many thanks steve

 

There were 5 production changes during the model a years. You could have damaged them if you dis***embled them incorrectly. Some years came apart differently.