I have been trying to decipher forks, triple clamps and what parts fit what. At some point might post up a cross section of them as a reference, but I am working on a variety of machines, What I have discovered and known is that there is a very confusing amount of part numbers and parts associated with all this.
My Plunger A10 does NOT have the casting on the neck like yours....... So I am wondering if your frame is later?
My rigid A10 does NOT have a steering lock casting either.
My rigid single B33, NO Lock on trees or casting on neck (1949 I believe)
But some of my later bikes DO.
Sorting top clamps and matching to bottoms. But I made up a list from parts books of all the clamps top & bottom 1948 thru to 1970, Singles and twins. Then same for forks, sliders and such.
Ill post up some pictures, or even a cross reference perhaps. Boggles the mind. Casting numbers and part numbers are not the same, or usually not. I spent hours today sorting thru, and found SOME rather conclusive reports on BritBike.com for the BSA and Triumph ones, but not for every application.
What is confusing is that SOME top clamps are missing instrument holes and lock holes, so indicates either early or competition models. Then some HAVE the castings for holes and locks but are not finished-machined or tapped.