The Ministry of Labour (works) was in 1923, 63 Argyle Street, previously in 1910 the address is shown as being occupied by, J Roberts & Co, Silk Merchants. Later in 1938, 63 Argyle St was the address of, Harris Bros, Tailors, Boilermakers Society, Society Of Woodworkers, Fred Wharton, Steward. Perhaps part of the PO was used before moving to Bridge St.
Images, 1923 & 1938 directories.