Mea culpa. I didn't realize that the first single carburetored version of the 300 series engine was designated M186, followed by M187 (twin carburetors) and 188 (triple carburetors), while the later fuel injected version was designated M189. The carburetored versions did have the spark plug in the block, not the cylinder head, whereas the M189 placed the fuel injectors where the spark plugs had been located and moved the plugs to the cylinder head.

I don't believe the cylinder block of the M189 engine was made of aluminum until 1960-61 and thereafter for all applications until the engine was discontinued in 1967.

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1973 Citroen DS23 EI five speed
Past vehicles;
1959 Thunderbird convertible/430
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang