With the subsequent passing of the Main Roads (Amendment) Act of 1929 to provide for additional declarations of State Highways and Trunk Roads, the Department of Main Roads (having succeeded the MRB in 1932) declared Main Road 373 from Terrys Creek along Sebastopol Road to the intersection with Lane Cove Road, and from the intersection with Delhi Road along Lucknow Road and Moore Street to the intersection with Longueville Road in Lane Cove on 16 March 1938; this was successively extended further west along Laurel Avenue to Epping on 21 June 1938, then along Carlingford Road to Carlingford on 17 April 1940.
This sequence of events occurred following the passage of the Main Roads (Amendment) Act of 1929, which enabled further designations of State Highways and Trunk Roads; the Department of Main Roads (which had replaced the MRB in 1932) designated Main Road 373 on March 16, 1938; it was then extended further west to Epping along Laurel Avenue on June 21, 1938, and eventually to Carlingford along Carlingford Road on April 17, 1940.