fal_shooter...
I purchased 2 sizes, the small ones Enco P/N 505-4543 and the larger P/N 505-4545. The difference in size between the two is great but needed in parts of the build. The larger clamp is great for holding the front/rear trunnions in place during fitting. I only purchased two of each size but would reccommend buying four of the small ones.
I do use the small ones for holding the lower rails but you need to add a small piece of .250" bar stock between the clamp and rails (I'll be showing that when I post the lower rail installation procedures). It seems that the small clamps hit the upper rails when trying use them alone to hold the lower rail and the larger clamp is to big to fit between the rails. Maybe I should buy the 2" clamps (in between size of the ones I bought) when I buy the additional smaller clamps. They might work without using the bar stock.