The BSA A10 has been known to have excess pressure build up in the rocker boxes. For racing a breather was taken from the rocker cover and vented to the atmosphere. During the Café race days Eddie Dow capitalized on this by making nice Finned Rocker Covers threaded to fit a breather. The finned cover above is of WEBCO manufacture, the USA equivalent to Eddie Dow.
The top cover is the one I modified to fit my bike. Note the hole for the breather is drilled above the centerline of the cover. This is done to accommodate the PCV valve I have fitted which just fits in the deepest recessed portion of the rocker box. I feel this mod works, as I don't seem to get leaks from the rocker box area, which is common on some A10s. The sound of the PCV valve working can sometimes be confused with a loose tappet adjuster as I soon found out!