Apologies for sounding rude! Basically what is now the administrative County of Hampshire... a misnomer as 'Shire' = county, was until 1959 the County of Southampton. Until then the county seat was in Southampton, but it was moved to Winchester. However there was also the County Borough of Southampton which had all the powers of a County in a unitary authority including registrations. Hampshire included the Usle of Wight until 1974, although they had their own registrations.
We therefore have:
HAMPSHIRE
SOUTHAMPTON CB
PORTSMOUTH CB
BOURNEMOUTH CB
all issuing registrations. plus the ISLE OF WIGHT.
Then in 1964 the town of Hampshire was granted a Charter as a City by the Queen in view of its size and trading importance even though it lacked a Cathedral [which Winchester and Portsmouth had/have]. Thus the trade plates would have been issued in the County of Southampton, and the 'Humber' has 0108 HO and the Ford 0106 HO, suggesting that they were contemporary trials trucks. I have seen Southampton CB trade plates, and some local dealers have had theirs up to 50 years I think! They have say '123 TR', as Southampton CB had 'OR' 'CR' 'TR' and 'OW' from memory...the Mayor's car was 'TR 1' which Tim Rice wanted to buy at one time!
I can only suggest that the trade plates were used by the MWEE at Farnborough, north County of Southampton/Hampshire, rather than London CC ones which the WD used.
Interestingly, 0108 HO was used in 1936 on the experimental Leyland Cub 6 x 4 triialled in North Wales, 0107 HO a Guy FBAX, 158 HO an AEC Marshall 6 x 4 and 157 HO Crossley IGL8. So these plates were well-used!
Just added the photo of the Crossley with 157 HO trade plate on the 6 x 4 thread. The original glassplate neg is stunning and would make an excellent framed photo as would the others that I have used in VRS articles. All were COMMERCIAL MOTOR magazine professional originals.