OK, no Revelation experts have chimed-in, so I'll attempt to deduce my own answer.
It has to be a 1964.
As I discovered earlier, the 20" barrels were only made from 1964-1968.
I read one one web page that Western Auto only contracted the Revelation 225's from 1962-1964. Savage 340C barrels were 18-1/2" in 1962 and 1963.
Therefore, if the above bits of information are indeed correct, 1964 would be the only year that Western Auto sold the 20" barrel version of the Savage 340.