Hi Donald,
Your #8 Do is PN 833.....not 633.  PN 633 is a wheat and corn stick pan.  Sometimes those numbers are hard to read.  
There is no listing in the red book for a 493 lid.  Interestingly, PN 491 is a #9 Dutch Oven, unmarked.....and PN 494 is a No. 9 Dutch Oven cover, unmarked.
It would certainly make sense then,  that your lid, PN493 would be an unmarked lid for a number 8.  
Since Griswold made an unmarked #9 DO and lid, and since you have an unmarked #8 lid, might it not be reasonable to assume the Griswold did indeed make an unmarked #8 DO?
I think this might be valuable info to forward to Dave Smith, perhaps with a pic if Donald would be so kind.  He might want to add this to his book.
Hope this makes sense to you, Donald.  Thanks SO much for sharing this information!