The substance of the SN chart and "bold face" note is apparently taken from the Whatley & Crawford W&C Scott book (page 74) wich has date ranges of 1897-1971. It covers both WC Scott and Webley & Scott guns showing the numbers assigned en-bloc to the Scott Division in Bold typeface. These correspond to the summary 400NXp gave.
Of course, Scott/Webley is known for it's contradictions and exceptions so the charts are simply a basic guideline. For example it is not uncommon to find "Proprietary" model (A&D type Model 400, etc similar to more modern 700 series) Webley boxlocks with extra bells/whistles and the W&C Scott and sons stamps on them. After all, it was one big manufacturing company after the merger and "parts is parts".
