This square-section O-Ring fits most Mocal sandwich plates including our Part Nos. 1236-3/4x16 and SET-SP1 series. The outside diameter measures 2.85" (2 1/2" nominal diameter) and the cross section measures 0.350" wide by 0.250" thick.
Older Mocal sandwich plates (made before 1990) take the thinner Mocal Old-Style Sandwich Plate O-Ring (Part No. 1233).