Here's an example of a clutch spring nut with the pips underneath the head (with acknowledgement to British only Austria) who are selling it, you can just about see the pip on the rightmost nut.
Generally these clutch spring nuts were used on unit singles such as BSA C15's. B40's & B44's, the idea was to stop the nut from turning and loosening, it didn't work very well.
A later modification was to drill a small hole through the side of the head so that two nuts could be locked with each other. This proved much more successful and is what I use on my B44 and I think what was used as standard on B50's.