That's a good question. I'm curious too? I've never looked at one side by side, but the 741 has a much smaller pinion stem then the 489. Maybe the gear area on the 741's pinion was a smaller diameter also and the clearance was made up by a thicker ring gear?
I'm sure some of the more experienced guy's can answer that question. Regardless, you need to count your teeth to be sure what you have.