How did SGC decide to add cards to a set checklist?

[SMPEP]

Okay, I may be asking a dumb question - but does anyone know what process the Standard Catalogue used to add cards to a set checklist? What was their quality control process for set additions?

For example, in the 2010 SGC edition, they added Walt Babare to the 1923 W572 checklist.

But ... there has never been a graded version of this card. There has never been a public photo of this card. Every advanced collector of the set that I know has this card on their want list. When you talk to these collectors and dealers, no one has ever even seen the card. Further, these cards were printed in 10 player strips, and there were already 120 known cards (a number divisable by 10) which is also a common print number for many sets of the era - so 120 was a pretty logical set total and 121 is pretty illogical (maybe even unlikely).

So how did they decide to add this card? Did they actually see it?

I know there are cases in which a new card is discovered and widely known, so it's not a surprise to add it to the list. But there are other cases, like this, in which there is no known explanation for the addition. How do I even know this isn't some T206 Magie type printing error by the Standard Catalogue?

Oh ... and if you have a W572 Barbare ... please feel free to show it. That would render my question moot!

Thanks,
Patrick