Looking for what criteria set the following Shimano rear derailleurs apart from one another.

Models: TZ50, TX35, TX55, TX75 -- They range in price from $10 to $20

They all have a max 34T capacity, 43T wrap, 6/7 speed compatibility, same listed weight, light action design, and direct mount or dropout mount options. The TX series all have "Smart Cage, Mega Range" compatibility.

Thanks, Paul