Motor Difference Question

I need to replace my Briggs & Stratton engine. My old engine is a 445677-0421-E1 and I have located a new 445777-0129-E1, but haven't been able to look at it yet. If I read the model number right, the only difference between these two engines is the output shaft designation (6 vs 7), but i can't find any information about what those number designations indicate. Does anyone know the difference between a 6 output shaft and a 7?

The crankshaft is the same on both engines. A quick check also says both engines supercede to the same replacement engine.