I believe it derives from the days of flags, field telephone code et al. This tradition is retained in the informal naming of the various callsigns in the Batteries (*ahem* Field Batteries), such as 'How 1' for CP 1, 'Jig' for the Sigs Sgt truck, 'X Ray' for the BC's Rover, and, of course, 'Romeo' for those heroic individuals, the OPs.
I once had a BK who always called the Battery Tiffy by his 'old school' callsign. The tiffy used to get most upset. Why? His callsign was 'Bluebell'.