Engine pad stamp 4118600 F0513RF
"4" 1964 engine , "118600" last 6 numbers of the VIN ( they were hand stamped with a gang punch and sometimes things just didn't go right on the line causing slips) , "F" engine cast in Saginaw Mich and assembled in Flint Mich , "05" May , "13" 13th of the month , "RF" 375 HP 327 with a manual transmission.
Sometimes there was a punch used that looked like an "I" for the number "1"
If they have a slip and punched it wrong or didn't hit it right they would just give it another smack or repair it. They were making engines to sell and not a work of art.
I hope I didn't lead you down the Garden Path.