If a person plays around with this stuff long enough, they can looks at those numbers and tell a lot about what it was meant to fit. With Ford's first digit being a decade code and the second being a year of change code then they just have to know what the other numbers mean. The 335 series engines were basically the 351 C and the 400. The 400 changed the family, ie 351M, early on and the oil embargo in the early 70s slowly killed them off.