One of my 65s had SO# stamped on top of the cowl (in front of the windshield), mostly covered-up by a piece of rubber molding. My other (pretty much identical) car had nothing at all in this location . . .
Don't remember for sure, so don't quote me . . . but I thought I read somewhere (years ago) that there were maybe 5 "secret" locations where they might stamp the SO#, and they'd randomly pick 3 of the 5 locations to stamp. Another place I think I've seen stamped is along the trunk drip rails . . .
Oh, and another little bit of trivia, is that they all use the character "I" instead of the numeral "1" in these locations . . .