Its the most common type fitted, usually found on original bikes or ones restored in the 1970's. As BsaReg says, the construction allows the inner bearing material to rotate, cutting off the oil if the retaining peg is absent or shears off. The very early engines had what looked like a white metal bearing in a steel carrier, in effect a one piece bush, then replaced by this type.
To remedy this, BSA moved over to a solid one piece bronze bush on later engines, and this is the best choice these days for replacement. They are available in stock sizes, and some enterprising folks will make you a custom bush enabling a well worn crank to be ground just enough to clean up, even if worn beyond a stock size, and also give a second life to damaged crankcases by boring the bush location, to allow an oversize O.D. bush. In fact it is better to extend the life of the crank by grinding off the minimum, installing an undersize bush and getting it line bored to the running clearance on the crank journal, rather than sticking with stock recommended sizes.
Even if just polishing the journal and installing a new matching stock sized bush, they sometimes need to be line bored....the bush closes down slightly when fitted. Plenty about this already on the Forum.
As this is the new toy, do a bit of research about crankshaft sludge traps, and make sure it gets a good clear out!!!!!!
Swarfy.