In Rick Ybarra's Boss 351 book, he states the early B1's had bellhousing C5TA-6394-A, and the late is D1TA-6392-AA. He is unsure when the switch occurred, but says his with a 135... VIN has the D1 bellhousing. My boss is a 141... VIN, scheduled for build Nov. 16th, actually built Nov.30th, 1970...