Ron,
It might be better in future to keep to the original thread, I think, rather than starting a separate posting. Not to worry, though.
The W5 number is also an engine number - you will find the explanations about early Enfield twin numbering given by Grabof54 (the REOC chairman) about a dozen or so threads down the page, interesting and helpful, I think. There are three threads to read, the second one is of particular interest in your case:
Royal Enfield & Indian twin engine identification
Clarification More on Royal Enfield twin engine numbers
Further notes on Royal Enfield twin engine numbers
The HDK number is the number of the original gearbox supplied with the engine, and should therefore be found on the top right of the gearbox. The HDKxxx series is correct for the '55/56 period. All you need now is the frame number, which should be stamped on the left side (looking forwards) of the steering head, i.e. ahead of the tank fixing bolt.