I'm thinking coloration on the reverse is due to exposure to environmental agents, not a missing clad layer. An accurate weight (to two decimal places) might tell us more.
Some good info per Mike Diamond that I wrote down:
1965 Quarters to present the clad weighs 1 gram on each side. One side missing clad will weigh 4.67 grams. Both sides missing clad will weigh 3.67 grams. Something that is good to know.
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