Proline Decorative Concrete Systems SL500A 24" x 36" OPA Locka New England Slate A Concrete Stamp. Opa Locka New England Slate is a two part system, with both an A piece and a B Piece. The purpose of multiple tools in a set is the open up the pattern so the eye does not see repetition. Each piece has slight variations in the texture for a more random look. New England Slate is the background texture. Also check out our line of Ashlar Slates for a similar pattern with a large selection of background textures.