Love the bomb sight Andy, and nice work to date.
As for the seat cushion, I'd guess it would depend on the type of parachute worn - if the seat pan is dished, it could be for a seat-type parachute pack, or it could be for a dinghy. However, if the seat back is at least semi-dished, or recessed, then it's probable that a back-pack parachute was used, as on the FW 190. In this instance, and considering the aircraft operated mainly over land, a cushion was probably used in the seat pan, again as on the FW 190, where it would be considered that a life-jacket would be sufficient in the event of a pilot landing in a river or lake after bailing out - assuming he survived the bail out that is!