Rear band pin seal

Hi all,

I'm building my first 727. Got all of the sub assemblies cleaned and rebuilt and I'm putting it back together. I'm assembling the rear band apply assembly. Go to look in my rebuild kit for the pin seal/o-ring. The one that was in it is square, profile-wise, but there are no square ones in my kit. The closest round one fits the pin OK, but the outside diameter is quite a bit larger as you can see in the pic.

I tried to install it, but it doesn't want to go. Lubed it and the bore up, tried squeezing it in with a small screwdriver, but it just seems too big. I'm afraid to push it harder with the screwdriver for fear of gouging it.

Is this supposed to be the o-ring I use, or was the kit missing the correct one? It's a 65 transmission, if that makes a difference.