Topps checklists are often wrong in this time period. It was evidently some sort of marketing ploy, giving kids a 'start' on the next series by having the checklists not actually correspond to what was in the packs. A 'you've already got some, you should keep going!' kind of a deal.
We know the highs do not start at 507; we've recreated the sheet and the 6th series stock variant clearly tells us which cards are in the 6th series beyond reasonable doubt.
This thread has more correct info on the highs than, I think, any other source online or published:
https://www.net54baseball.com/showthread.php?t=258947