Hmm, looking at the AIM, UPC M20-A1 has the following list:
3950376 - L6 (assume Saginaw)
3950378 - L65 & LF7 (assume Saginaw)
3950356 - L48, L34, L35 & LM1 (assume Muncie)
3946797 - Z28 & L78 (Muncie)
3950316 - M21 w/L34 (Muncie)
3946798 - M21 w/Z28 & L78 (Muncie)
3950318 - M22 w/Z28 & L78 (Muncie)