I'm a 901 user but I've recently tried a 510 carto on a
mod I made for that purpose. The flavor is a little lacking on the 510 so I started researching the kr808d-1 carto for my 901
mods. What I found out is that the 808 doesn't have a vent hole in the side like a 901 (I think it's in the connecter ring like the 510) so what you need the adapter for is the air hole.
The alternative is to drill a 1/32" hole in the atty connecter of the 901 below the
threads. Seems like the adapter would be easier. they cost $5.99 everywhere I've looked.