I had a few "Masterwork" pressings in the past with that same stamp: "L-O-N-E-L-Y" by Bobby Vinton (Epic 5-9791) and "Catch Us If You Can" by The Dave Clark Five (Epic 5-9833). Those must've been from another Masterwork compilation a la {Image #1704901}.
Added scan of side A with the stamped text "Masterwork". This single was included in the box "A Swinging Party! From Masterwork". This box included five comercial singles, currently available at the time. Apart from 4-43683, also includes 4-43625, 4-43673, 4-43678 and 4-43728. Front of box added.