It's almost certainly Bull ('I' Troop) you are looking for. I think crf might have meant him as it Bull's troop that, according to Mercer himself, had the battery composed of 5 1/2 inch Howitzers.
In the footnotes of his Journal Mercer also states;- "Ramsey wore the light-cavalry belt instead of a sash; Bull wore beard and moustache; so did Newland; I wore the moustache.......At that time the regimental pantaloons were pepper-and-salt, with straps of brown leather inside the legs and round the bottom, and a red stripe down the outside seam". Mercer recounts that he wore these overalls from 16th to 19th June.