I have seen much confusion over the numbering system adopted by Norton, and most online sources for decoding are erroneous. A very useful table can be found in Bruce Main-Smith's Norton Motorcycles 1928-1955. A full copy of this can be found under the "Norton Literature" tab above.
Prewar machines do not have matching numbers, frame and engine numbers incremented independently. Military models also follow this sequence. After the war, Norton restarted at 1000, prefixing the number with a letter denoting the year, and a number denoting the model, followed by the engine and frame numbers which match. For example, L14 65333 would denote a 1956 Model 99 Dominator twin, which would be stamped on both the LHS of the engine crankcases, and on the LHS frame gusset above the swingarm spindle. The frame was stamped on dispatch through the paint.
-BC