Hi Perry --
Your 77D Kennedy looks normal to me. Are you saying it is silver clad? Wow. That would be something!
1964 - 90% silver
1965 - 1970 - 40% silver clad
1971 - Date - normal nickel clad with 90% silver proofs
Proofs have S mint mark.
A jeweler with an XRF analyzer can easily determine metal composition non-destructively.