PSA type card slabs vs aftermarket
 

Is there a difference in graded slabbing and a 2 or 4 screw down holder ?
Do the holders actually cause damage or is it presumed ?
Thanks.

Screwdown holders without a recessed area can definitely damage cards through pressing. Some also allow moisture in over time where it gets trapped and steams the card in high humidity/temperature environments. You're usually better off putting cards in Cardsavers or penny sleeves/toploaders if you want to protect them the best.