Dear Spurtle
The photo is of the junction of Union Street and Forth Street.
The lamppost is MacKenzie & Moncur No. 101 according to Steven Oliver, who appears to be an expert on these sort of things. MacKenzie & Moncur had their main works in Balcarres Street but they also owned a foundry in Slateford Road according to Scottish Ironwork. Their work is scattered throughout Edinburgh including the rather magnificent Hothouses in the Royal Botanic Gardens.
Kind regards,
Stuart Gundry