Missing engine number?

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Hello! I am a newbie in the Ro 80 domain, I have just bought a wreck. It is a 1974 single plug modell with exhaust gas reaktor. I was removed the air filter and cleaned the top of the engine block, searching for engine number. All I have found is a flat zone paint in blue on the top of the intermediate element, between "cylinders".

Where else the engine number can be? If there is no engine number, what can be the reason for?

 

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