It becomes even stranger when one considers that this was a lowbridge bus. This batch of Weymann-bodied TD4 Titans were sold out of service in about 1950/51, although some Beadle-bodied examples were rebodied just after the war (Beadle again) and lasted until about 1958. The same style of Weymann bodies appeared on Bristol GO5G chassis in 1936 but were transferred on to new K5G chassis in 1938; these lasted until about 1955. Equivalent highbridge Weymann bodies were supplied to the subsidiary Chatham and District fleet in 1936, again on GO5G chassis later transferring to K5G.
Later in the 1950s, Borough Green (BG) had four lowbridge buses for the no. 9 and one highbridge for the 122. The rest of the allocation was single deck. Apparently a lowbridge would sometimes deputise for the highbridge on the 122. As the bus in the picture has "BG" clearly shown on the back, I suspect that this is what has happened here. I am wondering if, at the time of the photo, there may have been inter-working of vehicles between the 122 and the 18. I wonder, too, if the photo could be somewhere in Brighton, in which case the Weymann-bodied decker peeping over the wall could be an AEC Regent of Brighton Corporation.
I should like, if I may, to raise a further query, and that is as to whether BG at any time had more than the one highbridge decker and, if so, whether that could have appeared in Sevenoaks as a short working on the no. 9 between Sevenoaks and Borough Green (the through workings had to be lowbridge because of the low bridges at Platt and West Malling). I have one of those nagging memories from my toddler years of seeing what must have been a Short-bodied TD4 heading along the Seal Road in Sevenoaks in the Borough Green direction. I suspect that this must have been a no. 9 short working.
Incidentally, the road was later lowered under the bridge at Platt although closure in the 1960s of Offham level crossing caused the route to be altered, involving yet another low bridge in Seven Mile Lane.