There was an interview with Petrov (in the 90s or 00s) when he spoke about this point, and the interviewer said that in 1983, the US plans for a preemptive strike started with a "beheading strike", in which a few missiles would be sent to hit the Kremlin and a handful strategic military bases. Then the US would wait if the Soviets were still twitching, in which case the second, more massive round would've been fired. Petrov admitted that, if he had known that, he would've decided differently on that fateful day.
(Don't have a direct source link to back this up, sorry. My source is one of the earlier episodes of https://alternativlos.org where they talked about the Cold War.)