Yesterday I was looking for the same info. Have found out there were two sizes and two MM numbers;
MM #10A79 2 1/2" bore
MM #10A80 3 1/2" bore
They take what"s called "snowman" figure-8 end seals, accordign to Mrs. Welter at Welter"s Farm Supply in Verona MO. She"s got them.
My smaller one doesn"t quite have the big end cap yours has. Mine has the 10A79 number cast in the side. You might have the 3 1/2" bore.