Hey need some input again

Went to napa today to get new cap and rotor and they told me my cap is off a 56 or previous distributor but the rotor was 57 was there a half year in production maybe or is it a older distributer was wrong rotor or somethng else ? Im in process of getting it running and previous owner is clueless maybe you guys know something napa dont any input helps i know for sure a57 cap is to large is there a way to id dist. Without pulling it just to refresh its a 57 f100 with 272 i thought motor was original but now second guessing it