To be honest, I'm not convinced it's right for a Southern (or ascendants) loco at all and may only be representational.
Laurie does do the Drummond set; I supplied the research,
@SimonT of this parish did the 3D CAD work and
@Nick Dunhill did the test build on the couple of T9s he's recently built, the build up of the first sets starts with this post:
Nick Dunhill's 7mm workbench. | Page 16 | Western Thunder. It differs in more than just details from the example posted. In the same way that (for example) you wouldn't expect motion from an LMS Duchess to fit a GWR Castle, yet both have Walschaerts/Heusinger valvegear operating to similar principles.
Eccentrics and eccentric rods vary enormously between locos with inside Stevenson motion, depending upon whether the loco has straight, or rocker-shaft driven valves, whether the inside or outside rods are forward or backwards, whether the expansion link moves up or down for forwards gear, what valve type is driven by the 'gear, whether there's an axle in the way, personal preferences of the designer or workshop, left-hand vs right-hand lead, etc, etc. So, if you're going to model the inside motion it's always worth trying to find a suitable Arrangement drawing - assuming one exists. Then all you need to do is pitch off the dimensions and build it according to the drawing and it'll be something like correct.
If you want to get into the theory and practise of valvegears then the Reidinger and Lake book I referenced a post or so ago is a good starting point and I think has been recently re--issued by Camden Steam Services (it is, see here:
Valves & Valve Gears for Steam Locomotives · 1940 · | Camden Miniature Steam Services). For a less detailed look that's perhaps better for us modellers I recall the HMRS did some papers that explained the differences in layout of various valvegears.
Steph