I'm getting the same thing Martin, only on 30 shots.
By the 4th diagram I can't wait to finish and probably rush them subconsciously.
I had to shorten my sling one notch to shoot outdoor as I reckon the targets were a little higher than indoor. I also have the cylindrical knurled stop about Ø28mm.
I'm thinking it may be the sling pressure on the back of my hand as Graeme just said.
I might have a go at making the handstop longer so the sling doesn't press against my hand.
The SKP one, Graeme, isn't a good picture and is hard to see how deep it is.... or even how it fits!?