The numbers are indeed serial numbers. I believe they started with 1001, and went up from there. Both are fairly low numbers. If you can get the total production numbers of the New Yorker for 1957, you could see where those engines fit in production-wise.
Also take a look at the casting date on the top side of the block next to the valley cover, it will give you the date the block was cast. I read somewhere that all of the 392's, both '57 & '58, were cast in the same year, at least the ones from the A-1 foundary. Don't know if this is a fact though...
just passin' thru