were build sheets put in these PUs ? I kinda glanced under the seat and I didnt see anything obvious,but didnt spend much time cause I wasnt ready to roll around in the crap on the floor.

 

I would assume there was one originally, because 1961 and later all had them.

They were either stuffed in the seat cushion among the springs or placed under the floor mat (usually on the right side) at final assembly.