[papajam]

TorW has it right. The 'S' designates that the engine is either an emcon engine (emission control) or a standardized (for emission control) engine. A standardized engine differs from an emcon engine only in having no emission control carbs.
The 1600cc 00536 'S' engines differ from the non-S engines in having different distributors, ignition timing specs, front pulleys and the exhaust cam advanced from 102 degrees to 111 degrees. The emcon engines also have different carbs and jetting. These engines are also marked with the letter 'D', 'E' or 'G' on the front of the cylinder head.