Remington and Winchester use 385 grain pills in their sabots for 12 gauge, 260 grain in their 20's. Hornady uses three different bullets in the 20 gauge stuff but all are 250 grain while the 12 gauge Hornady ammo comes in either a 300 grain or 325 grain. Federal sabots are in line with Winchester and Remington bullet weights. I have a 12 gauge Winchester partition gold and a 12 gauge Winchester BRI sabots here that have the crimp removed so the bullets/sabots can slip out of the case, both using .50 caliber pills, the BRI a "waisted" lead number that weighs a full one ounce. In both of those the sabots are very thick walled, appearing to be much thicker than those in your pictures so I'm inclined to think along the 20 gauge lines. In rifled shotguns the rifling in the barrel is not near as deep as rifling found in muzzies or centerfires so sabot compression in a shotgun may be way more radical than in the blackpowder world.
In any event I like to know the exact source of what goes down the barrels of my guns so the white sabots would be chucked and known sabots paired up with the bullets since I can't find any factory ammo using the speer 270 in sabot shotgun ammo.
