MAR likely meant "Marine". WT was a water tested block. Yes, Max Wedge and Marine engines (and other special engines like Race Hemi's, 300 Letter car dual quad 413's, etc.) were built at the Marine facility.

The large size font stampings (regular production engines had a smaller font) are also a tell tale sign of a Marine facility assembled engine.

Max Wedge block castings were not unique to just Max Wedge engines, the same castings were used for Marine engines, Truck engines, Industrial engines, etc.

Bottom line? What you almost certainly have there is a boat engine.