I'm not an expert, I do think the 901's came out before the KR8's. I think a lot of vendors supply both the 901's & the KR8's.
I really don't know why the 901's & the KR8's battery
threads are the same.
I think a lot of the E-Cig manufactures felt that there would be a high enough demand to produce adapters for many types of E-Cig
batteries, as there are many different types of adapter available.
I know very little if anything about 901's as I started were you did with the V2-Cigs KR8's style E-Cig.
I did move to the eGo with a LR atty & drip tip right away, and I have not looked back.
I would never order anything from V2 Cigs ever again. I've had nothing but trouble with that vendor.