The BB is an EBB or Eccentric Bottom Bracket. They are used to tension the chain if you're using the frame as a single speed. They were first used on tandems to tension the chain between the stoker and the driver. Personally I think they are great for singlespeeds with disc brakes as they don't need any adjustments of the brake when using a different chain ring or rear cog or taking the wheel in or out.
That's what I thought. Does anyone recognise the make of the frame?
The reason I came across this was I was going to buy some wheels of him, and I noticed he sells an awful lot of stuff. He must work in a bike shop.......