Mike, greetings. I am not quite sure what you thought I meant in my last post, but I posted the image of the Scots Fusilier Guardsman from the 'Crimean Heroes' series to demonstrate what is indeed the more structured cap that the Foot Guards retained from the period before the Kilmarnock was ordered, (which I think equates with your 'P1830') and which appears to have had a band of distinguishing colour for the Grenadiers and Coldstream regiments. I believe the Scots Fusilier Guards did not adopt a band of dicing on theirs until circa 1838-39(IIRR).
As you rightly divined, my intended point was that, given Brownrigg's Foot Guards connections, taking the distinguishing band as a cue, the cap on the RH boy in the photograph might
be a Guards forage cap (evidently Grenadier) albeit a fairly battered example.
Thinking a little more about the matter, given the evidence of structured caps with bands of distinguishing colour still being worn by other corps (as well as more 'classic' forms), there might be other candidates, depending on your interpretation of the original form of said battered cap.
I warrant, the tinted image in the previous post, if viewed in its original monochrome, would nonetheless show a band of distinctive colouring. Posted in haste, the caption was not mine.
Perhaps on another thread, I'd like to know more about the 'P1830' label. We do have watercolours by Michael Hayes from 1837 of the 46th Regiment in Dublin wearing your 'classic Kilmarnocks,' so although we know of regiments continuing to wear older forms, or foreign service 'special editions', into the 1850s, Barthorp's 1834 date may yet prove to have basis in fact.