After 1918 hammer guns were stamped with an H and the first serial number was H201(from Records) and now they all had Royal Steel barrels. They did away with the F grade because now the serial numbers were mixed and the hammer guns were given the prefix H. Catalogs from this period listed HH for 10 gauge but there were none made after 1918.
My theory on the different grades of barrels is that in the "white" all barrels look the same and I believe they were put in the wrong bins. I do have one serial number of one barrel having Best Damascus and the other Chain Damascus. When assembled and given serial numbers then blued and etched, the pattern then comes out. Or a very slim possibility that they could have been Special Ordered.
I have 3 Chain Damascus barrels in over 600 serial numbers. I also have 3 that started out as an F grade and one wound up as an Ideal Grade with Star Damascus barrels and another wound up as a Specialty Grade and sold again both times through a Julia Auction, the last was a Trap grade that in the records state it was an F grade sold through RIA.