Yep, they don't just slide out.Retainers would be on the underside of the carbs. In fact I think it's the grub screw & locknut juuuust peeking out on the right side of the bottom photo (next to the Leatherman tool, covering the X in "SpaceX").
The white plastic caps cover an adjusting screw that doesn't need to be touched (for 90% of applications).
If the venturi is well and truly jammed and corroded in place, you can make your own puller. Details and photos are in the invaluable Des Hammill book How to Build & Power Tune Weber & Dellorto Carburettors. It's basically two pieces of bar stock and a threaded rod or bolt between them. One bar sits across the throat opening and the other bar presses against the venturi. The trick is that you need to contour the ends of that bar to match the radius of the venturi to prevent damaging it. And then you need to make a different one if you need to remove a different size.They slide out. They do make a puller, but have never used one.
Sorry, I overlooked this question earlier. Your second thought is correct -- if they had only one groove and one hole, there would be a left hand and a right hand version. Double the holes and grooves makes them universal.The new venturis have two grooves and two holes in them. Are there protuberances poking into both holes, or is that to make them common between both throats?