Yes GB, Martin's airbox , although being the correct shape (almost?) is not the right one.
My filter is glued to the backplate and I have never risked taking it off in case the whole thing crumbles. Looks a good bit of 1950's glue, so thought it was standard. Haven't got an owners handbook, so don't know the recommended cleaning procedure. Contemporary vehicles had wire gauze filters which were usually washed in petrol and re-oiled, but washing the filter here in petrol seemed to me to be a bit risky in case the glue dissolved. So maybe the P.O. did the work with the glue.
Martin,
The rubber air hose connection to the carb needs to end up on the centre line, in front of the vertical frame tube. A closer look at the pictures tells me the airbox is not the right one, and I reckon spanerc is on the money. The correct air box has an oval aperture facing towards the oil tank, in front of the frame tube. The connector hose fits in here, then bends at a right angle to join to the carb.
Yes, more pictures would help.
Swarfy.