The thumbscrew markings are correct... for an early enfield converted trials rifle. I have a very early rose brothers bracket that has that same stylized "D" (with broad arrow) which is actually an EFD and superimposed to that D. I believe these were found on early trials rifles converted by enfield to sniper pattern. While the brackets were produced at Rose Brothers and Daglish, I read somewhere Enfield produced the remaining parts (scope pads, thumbscrew knobs). I don't remember where I read that, so hopefully someone else can elaborate.

Attached are pics of the thumbscrews with efd/broad marks arrow on my trials rifle.