Aircraft within squadrons often changed their individual code letter, for various reasons. It may be, for example, that BH-K went in for deep service or repair, and a new aircraft became the new BH-K. When the original ( BH-K ) emerged from the hangar, it may have then been allocated "N" as it's individual code letter, becoming BH-N, possibly replacing former "N" lost on operations or damaged beyond repair etc.