The correspondence suggests that this was an Edinburgh-based law practice, with the majority of letters emanating from city addresses.
There are 2 volumes with information on the spines showing:
Letter Book No. 3 - 18th May 1956 - 21st Aug. 1956
Letter Book No. 5 - 9th Dec. 1956 - 31st Mar. 1957
The volumes came from The Solicitors' Law Stationery Society Ltd., Hope Street, Glasgow. They hold reference numbers: No.5354 Date.1956 & 4525/57