Each size carrier is for differenr size gears, such as 2 series will hold all the sizes in the 2s such as 273 and maybe a 308 max/ 3 series will hold up to a 373 and maybe a 410 with an adapter, 4 series will hold from a 373 up to 488s and so on and so forth, I have a 4 series 12 bolt posi and 410 richmond gears for sale if u are interested, Good Luck L89SEDAN