I recently was given a 7.5 mercury, runs well, by a fellow who has owned it for a long time.  The serial plate is missing, but on the side of the engine block it is written:  SN 200275.  Above this it says 1971.  That's it.  no 7th digit.  Any thoughts would be appreciated as I'll need parts sooner or later.  I really want to know what serial # or even a close guess @ the year to get parts later on.