I've seen both burner tubes from when I've picked them up for rehabs.
The 7506’s center tube is positioned horizontally just like the other two burner tubes, and the crossover tube lays flat on the end of the tube.
- this one has flames in the middle of the tube
20428 seems like it matches what came off of my Gen 2000. The center tube is vertical with the crossover tube running through it.
- Mine have the the flames on both sides of the tube (instead of just the middle).
Not sure of performance, I kept the 20428 on mine because I figured more flames, it may run hotter? But I'm sure the btu label shows its the same heat. (haven't compared). Ultimately, I don't know if there is a big difference.