I just took these out of an October '69 built '70 'cuda with factory R22 & R31 (a single speaker with R11 would be the same speaker on the LH side)
both were mounted with 3 screws - - don't know if that's how it left the factory or if one fell out of each speaker

1970 cars with STEREO (AM/FM or AM w/8-track) had two rear speakers VS. one speaker for the mono AM radio - they used the same R31 code for rear speaker(s)