The earlier B40's used the same carb size as the C15 and were able to use the same filter arrangement and connecting rubber hose. However, in 1962, they changed to a larger carb and at that point had to change how it connected to the filter - connection by rubber hose was no longer possible. Instead, they used a metal adaptor screwed on to the end of the carb which projected through the rear panel to the separate filter. My B40 is from 1965 and uses this arrangement. If the filter was still attached to the rear panel as per C15, it's difficult to see how the adaptor could be fitted. The parts book from 1965 is simply wrong and shows the much earlier rear panel and filter arrangement pertaining to 1960/1961.
I can't remember where my filter came from when I restored my B40. However, if you are careful, you can detach the filter from a C15/ early B40 rear panel and use that as a separate filter to plug into a suitable adaptor for you carb.