As I remember it, the:
Sten magazines have a narrow 'U' shaped opening at the rear to accommodate the narrower rounded feed horns of the breech block whereas the;
Sterling magazines have a slightly wider squared recess to accommodate the squared-off, wider and more efficient feed horns.

Surely, with the availability of Sten and Sterling magazines this is all academic. Just use the right magazines for the right gun...... Like using petrol in a petrol car and diesel in your old works diesel van