I’m not an expert on Bomber Command kit, but just as an “enthusiast’s guesstimate” I would think that A) a Nav’s bag would be a standard bit of kit for all aircraft types and the B) even in the heavy bombers the Nav wasn’t always at the table. On the Lanc there was a folding “second Dicky” seat behind the pilot and the Pilot’s headrest could be flipped down as something to lean on, so a Nav would sometimes sit in this position when D/R  navigating in daylight, at low level etc. 

This is how the BBMF Lanc Navs work today, sitting behind the Pilot rather than using the table.