There were a lot of changes taking place at the Robertson factory going into the 1930s.
Charles B. Robertson (J. R. Robertson's nephew) was president; he died December 3, 1931.
J. R. Robertson returned as president. He died July 15, 1935.
Clarence Mather (formerly owner his own boatbuilding firm, and at Toppan Boat and Engine Co,) was foreman between 1934 and 1938.
Arnold Laskey assumed ownership of Robertson in 1937, and operated to at least 1955. He died in 1959.
Whether any of these changes would explain the use of this serial number style is not possible to say at this time.