Wanted-1959 Pontiac rear back seat with center cutout for rear speaker.
 

Not sure but maybe only bonnevilles had the rear center speaker but maybe starchief and catalina also and believe only on a 2 door and cant use seat from a convertible cause it is narrower for the convertible top scissor arms to fold into the armrests

Only the Bonneville had the rear seat armrest. I'm pretty sure 59 & 60 are physically the same, but 4dr is different from 2dr. I also believe that Buick and Olds 2dr seatbacks with the fold down armrest are physically the same.