The manual states that " All fresh water cooled engines have had incorporated within the exhaust manifold (or upper portion of "S" type riser if so equipped), a thermostat to control the exhaust manifold temperature. " Then it shows the diagram. The thermostat is part # 2847431

Also regarding engine overheating or exhaust manifold overheating ( M273 & LM318 with single 1'' water inlet)
"When engine overheating is encountered, or a condition where exhaust manifold overheating fluctuates from side to side, a restrictor installed between the exhaust manifold and the exhaust elbow ( or between "S" riser and exhaust manifold where risers are are used) will stabilize the cooling system. "
It is essentially a washer .94-1.10 in diameter with a .35 hole in the center and is .30-.35 thick. These are the exact dimensions in the manual.

It continues: Restrictor is to be assembled into the machined recess around the water outlet passage in the exhaust elbow.
In emergency the flanged end cover from exhaust manifold thermostat P/N 2847431 may be used as a restrictor.
In cases where an old style exhaust elbow without the machined recess is used an additional gasket, P/N 2847432,
assembled between exhaust manifold and exhaust elbow should be used to locate the restrictor in the correct
position.."
Signed J.J. Bartos

And another pic..