Ref #4 above and “KW” possible code.
There is a good shot of a 425 “Alouette” Sqn Halifax III at Tholthorpe in 1944 on page 405 of C H Barnes’s Putnam book on Handley Page. It shows the Preston-Green single 0.5 ” Browning ventral mounting well.The same shot is in John Rawlings’s “Bomber Squadrons of the RAF” p244