Marks one to three were designed, made and rejected. MklV, with its concealed, whole cane butt and 1.5lb test-curve, was just the bizz and Richard Walker made a few up for himself and his friends; their concensus was that it was spot on for carp, but they'd like a lighter version for tench, barbel and such, and a beefier one for pike, salmon,and heavy-duty carping. Walker subtracted 10% from all diameters for the light versio, and added 10% for the stepped-up. Both worked, straight off the drawing-board. I THINK the lighter version was called the "Avon" as a marketing ploy when B.James put it into production, because the test-curve was about the same as their "Wallis Wizard" clone, the "Avon Perfection", although the MkIV Avon was shorter.
The story of the involvement of J.B. Walker's (no relation to **** Walker, and in Hythe, Kent if I recall aright)and B. James of Ealing is complicated and I've forgotten it! But J.B. Walker sold kits to make up yourself, and B.James sold ready-made rods. (Which is why I assumed that your Dad's was built on a J.B. Walker blank. I could EASILY be wrong!)
The highest-value rods are those made by **** Walker's own hands; then B.James rods signed by Walker in indian ink; then B. James with the signature on a varnish transfer, of which there were several slightly different versions (handle end shapes, whipping colours, etc.). Apparently , those on blanks built by Bob Southwell are rated the best of the production models, but I've no idea how you tell!