I think this "Honorable Discharge" letter was written much latter than 1978.
Check the differences between the typewritten text and the normal text. This letter appears to be back-dated to 1978. The specifics to Kerry are typed in.
Why was it back dated? When was this actually typed?
At the bottom, it says
"NAVPAERS 1926/2 ?Rev. 3-77)"
"Part 4"
You might be able to use that to to establish a date, but my guess is this isn't the original discharge letter.
But, what happened to the original discharge letter?
"NAVPAERS 1926/2 ?Rev. 3-77)"
Revised the form in 1977