This is an oil pump. Yes I know it’s not in the engine, but I want to point out something take for granted how easy it is to jump on a bike and just ride. This early 1920s BSA featured what is known as a total loss oil system (as well as featuring no headlight as it was only an option!). Without an automatic pump system mounted in the crankcase, it was up to the rider to watch the oil contained in the sight glass, and pump as required. A more vigorous pace called for more vigorous pumping, quite a task on the Australian roads of the first half of the last century. Next time you take your modern machine out, enjoy the ability to just twist the throttle and go, without worrying that you are responsible for whether your engine seizes up or not.