These were used on some pre-war lightweights, sometimes with the typical Amal hinged clamp, as here or alternatively mounted on studs welded to the bars. I've always assumed that they were a cheaper, economy lever. They certainly don't look as weather-proof as the standard type.
Hi Rik, I was just looking around for a slip ring and came across this lever on ebay. Never having come across this type of lever before I just got curious.