[BillyCoxDodgers3B]

That might very well explain it.

Is it best to attempt to flatten these out very gently over time before submitting, or just leave them be? If it doesn't flatten, don't slab, anyway. They're over 130 years old and I have to imagine that most are brittle after so long.

After handling so many of these, one would think the companies would know which ones need to be kept far, far away from their slabbing machines to save everyone a similar headache.

...but it doesn't explain why nothing happened to the OP's when originally encased in its former slab. Is there more free space between the SGC tuxedo sandwich? Did you notice any warping in the card when housed in the SGC slab? Yes, it's really too bad you didn't retain a photo. Have you scoured the internet on the off chance that an image of it can be discovered?