Grenadier Guards, around 1830, that oversized and imposing style of forage cap was common across all ranks and regiments of the Foot Guards at that time, whereas it was seen only on officers and battalion HQ staff sergeants in regiments of the line. It was invariably worn by officers with their undress frock coats. I agree that the 23rd had a bastion loop on their collars during the same period. By 1850 the cap shown had been replaced by the low crowned forage cap with its flat horizontal and spade shaped peak in the line, while in the Guards a soft crowned early version of what evolved to become the 1880 forage cap had evolved into use, recognisable by its so called drooping peak, bound around its edge with gold lace. There are several contemporary pictures of subject forage cap in page 6 of the thread titled ‘Grenade Fired Proper’:
https://www.britishbadgeforum.com/fo...t=14757&page=6
They continued in use until at least 1850 (see drummers below). Notice the chin strap, also distinct in your image, which was worn down when on parade. As I’ve mentioned previously the Guards were unique at that time in having two types of undress headdress, a forage cap, and a folding field cap. The badge on the forage cap was brass and that on the field cap cloth.