Recoil Rob, I believe you are correct, decorative at this point. Relation to Rigby flats I can not say.
Waterman, the 360 #2 is a fine old round, low pressure and great sectional density. Holland never chambered in it preferring the .375 variants.
Bob, I love the looks too and plan on contacting Mr. Silver regarding Holland style mounts.
Martinibelgian, I have no clue what kind of tang sight that actually, and most likely will not use it.
Mr. Petrov, almost every action in the books seem to have started out life as either a .256 or a .303, both great rounds but I have a .256 and .30 super flanged. Many of the guns were rebarreled before they ever left the factory and many more were rebarreled afterward by Holland and by others. I plan on a custom in the Holland style for this rifle with an upgrade in engraving to a style that was used by them. It should be a fun build.