Did 1970 A-bodies have a buzzer relay for warning of the key being left in the ignition or the headlights still on like B/C/E-bodies had? If so, did they use the same part number as B/C/E or was it a different part number?

Yes my 70 340 Duster has the key in buzzer only. Yes according to my 70 parts book 70 is the same between A and E body 3420884 and 3420885 for B body. 71 had different part numbers for A body , B and E body were the same. Same switch for both years and all 3 bodies.

70 key in buzzers are one year only piece, being triggered by positive. Since 71 was changed to a negative source.

70s also worked as a lights on buzzer adding a diode on the circuit. Harness get a plug provision for that.

Usually the difference between bodies buzzers is just the bracket shape, but the inners uses to be the same, so if you need a replacement and can't find the exact piece, you could change the inner gutters from one housing coming from a diff body to the exitant on your car. Piece of cake.

My 70 Dart does not have. a working buzzer. I don't have access to the car at the moment to check so I don't know if there is one in the car that is not working or if it is missing. I have one out of a 70 B-body so I just curious if it was the same one that my Dart should have.