... and those are indeed triple-headers!Without knowing where this particular picture was taken, I feel quite confident that this streetlighting installation was certainly not unique. In Bury, Lancashire, this same lantern / column combination could also be found, presumably on thoroughfares where high lighting levels would be required.
The use of both sodium and mercury lanterns on the one column might indicate that the full load would have been be applied from dusk, with the mercury lamps being switched out from midnight onwards.
I would recommend an excellent publication "The colours of Greater Manchester" by Michael Eyre and Peter Greaves. This book celebrates the wonderfully varied liveries of the former municipal bus undertakings, which could be found in what is now Greater Manchester. Whilst the book is splendid at illustrating the variety of vehicles in use at the time, it is also ideal for capturing plenty of long vanished street lighting.
Below is a scan from this publication. It shows a single deck Daimler Fleetline with East Lancs bodywork, outside the old Bury Market in May 1969. To the rear of the bus is one of those unusual columns, although similar pictures of Bury show they were to be found in other parts of the town centre. As already indicated, the Revo Horizons and Hyperion lanterns can clearly be seen.
The photo has been credited to a Mr Geoffrey Morant.