As far as I can tell from Bailey, the bags were probably just the standard light infantry cartridge pouch. As for the horn, unless one has been identified since Bailey was writing there aren't any known to exist, and the only thing we know about them for sure is that they didn't arrive in a timely fashion and that Ferguson had to prod some people to get them to his unit. Bailey hypothesizes that they had a spout throwing a fixed charge, but that is not known for certain.
We don't know what the British light infantry horn from the period looks like, nor, apparently, the horn issued with the Baker rifles. Really not much to go on.
