Uncle! The fact that there are numbers that don't match the beam is convincing evidence.
J.R. Robertson Co. was incorporated in 1927, And JRR's nephew Charles (who essentially founded Waltham Boat & Canoe) was President. Charles died in December, 1931. JRR became president, and himself died in July 1935. Charles was probably the main shop superintendent, so the change in serial numbering may have been made by whoever replaced him (one possible candidate is Clarence Mather, who had is own business dating back to the 1890s.
Arnold Laskey took over the business in 1937, who is listed as manager through 1955. He died in 1959.