From what I can tell 4 types of carts fit a 901, The thinner whistle type or cigarillo type, are the 901's. (Called the "true" 901) These usually come with your kit.
The fatter cigarillo type is what comes with the 4075. But I think it depends more on where the supplier orders from. I got these when I ordered some extra atomizers from a different supplier then I normally use. They were the "true" SLB 901 atomizer so it seems to be a crap shoot.
The orange round carts are the original 901's, they still come with some of the white 901's. (see east mall for a picture)
The round black carts are for the Janty Dura but fit the 901..a little bit loose, but not bad. ( see the janty dealer or Diet smokes for picture)
I have the Dura's for hubby who perfers a round cart, I just put a sticky dot on the edge to keep it from slipping out. Some fit perfectly, most do not. He also has the orange round but they look ugly with a black 901 even if they do fit.
They do hold more then the cigarillo flat carts but I don't like the feel in my mouth and I can't grip them with my teeth. The Black are harder and have flat ends, The orange are softer and have rounder ends.
I did several searches but couldn't find the thread that has pictures of the 2 different 901 cigarillo carts.
before you order send a P.M. to the dealer to ask which ones they have. I have always recieved the thin cigarillo when I order a bag of them.
Hope this helps.