The 70 Stage car that I am restoring is a October car. Wonder how far back from there is within the range of " possible ". Duane would probably know. I should, but can't remember. Lol
If I am not mistaken the range is 5 months but I would still check with someone that judges them like James, Dave, or Duane to be 100% sure
On the more common parts I'd think a realistic window would be measured in weeks and not months. I bought an NOS 1100774 not long ago with a September date for my 10E bodied '70 and felt like that was really pushing it but just couldn't turn down all those little Delco markings/date codes on everything. The places that build them like that for the Concours Corvette guys charge around $350 & that's supplying your own core.