The rim doesn't prove the metallic content of the outer cladding. It could be 90% silver rather than copper/nickel.
It could be, except that the has Mint never issued anything with that composition (90% silver over copper). It's one thing to suggest that a leftover silver planchet got mixed in with the clad planchets in 1965; it's quite another to suggest that you're holding a coin struck in a composition that's never been used. Far easier to believe (and demonstrate) that the outer copper/nickel clad layer can sometimes age to look like old silver. I've seen it myself.
I'm not suggesting that that is what I have, only that the error (whether intentional or not) is possible and looking at the rim does not disprove it's possibility. Easy enough to disprove with an acid test, though someone could have silver plated a 66 as well. My coin has the correct weight for a normal clad over copper quarter. It also has the whitish appearance of worn silver and toning to match that most all other clads do not. It's weight pretty much disproves any possibility of it being clad in silver. You could also do a specific gravity test or swing it on a metal detector to compare readings with other clads to see if there is any difference in the composition of the outer layers of the coin. Also, the 90% composition has been used, it's coin silver, just not on the outside of a clad quarter, that we know of, yet.
Btw, the coins that do appear silver after 64 are only 65 and 66, that I have seen. What would explain this?
The 40% silver half dollars minted from 1965-1970 are clad coins. They are similar to the cupro-nickel clad coins, but the cladding is silver instead of nickel. I can't remember, and I can't seem to find the exact composition of the cladding though. The core is copper and the cladding is silver, but the overall composition by weight is 40% silver. The cladding could be 90% silver. There were also 40% silver proof bicentennial quarters. I don't know offhand if they are clad coins like the 40% silver halves.
The problem is...you can clearly see the copper on the rim of this coin. That alone is proof that there is a copper layer within your coin. The 90% silver coins were a 90% silver/10% copper alloy...there was no layering, it was one solid metal. The 90% silver coins cannot have the copper layering we are seeing with your coin. Now, the clad coinage that began in 1965 can sometimes have the nickel folded over the rim to make it appear that there is no copper layer. But...there is still a copper layer inside. This is when some people start to confuse a clad coin with a silver one...because they can't see the copper. So...on a 1965 quarter, if you can't see copper on the rim it doesn't necessarily mean it's silver but if you can see copper it means with 100% certainty the coin is clad.
I didn't say that it wasn't clad, which it obviously is. I was talking about the metalic composition of the outer layers possibly being something other than cupro-nickel, like 90% or possibly 80% Ag, which could explain it's silver appearance.