Before paying for a copy of AP 1374 identify the magneto type and what you want to do to it.
If you want to overhaul a magneto circa wartime period you will need Volume II Part 3 Instructions for Overhaul. Remember Air Publications evolved over time with AL’s. (Amendment Lists) Pages were inserted and removed depending on the equipment current at the time.
If you post what BTH magneto you are interested in I will check my copy of the Vol. II to see if it is listed.