A. G. Mason sewing machines were mostly manufactured by the White Sewing Machine Company from 1903 to about 1916 when it was acquired by the Domestic Sewing Machine Company. Please check this link for information on how to download the manual:
Here is the link for White machines if you don't see yours in the A. G ***** *****st.
Mason made only 3 models: the Rotary Special, New Queen, and Golden Star.
Please let me know if you have questions.