The engine numbers start with the first two numbers being different; N600 E10- - - - - is an early Sedan engine number with a constant transmission. N600 E15- - - - - is a later (after 1 April 1971) Sedan engine number with a full synchro transmission. N600 E20- - - - - is an engine number for some Z or Coupe models sold in Europe with a constant transmission. N600 E25- - - - - is for the Z or Coupe with a full synchro transmission. N600 E50- - - - - is to a Sedan having an Automatic transmission.
The internal part for the cases of the constant and full synchro transmissions is different. The engine for the Z or Coupe have different parts from the Sedan and looking at the two; Sedan and Coupe parts manuals will show this. The full synchro transmission started in 1 April 1971, at the same time as the Z or Coupe production.
Many parts are the same, however many are not. All sedan chasses numbers are sequential the engine numbers don’t follow as they are made for different locations and markets, as are the Z or Coupe chassis numbers.
This is why knowing your chassis, engine and date of manufacture are important when looking for parts using the factory parts manual.
Bill