Hells teeth - that's absolutely gigantic, are you still using the 60W iron or do you have a "super model"?
In best Reginald Manning Wardle spotter voice, I was re - reading my MRJ compendium the other night, like you do, and Don Townsley showed a picture of "Aldwyth" and the internal view of its bunker which revealed a sort of half circular "cut out" which extends the footplate and effectively divides the bunker into two. He made the comment that a lot of modellers "get this wrong" which left me wondering about the internals of (my) bunker which looks just like yours, albeit with the cab back sheet sitting on the back edges of the sandboxes.
Does Professor Dale have an answer to this fascinating conundrum of cab interiors from the late 19th Century or should I get out more?