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You're correct, any sort of optical grade mirror is usually surface coated instead of back-coated to avoid refreaction effects and other problems from having the light pass through the glass before being reflected. (IIRC you could potentially get double images in an application like this; a faint one from the reflection of the glass surface, and the "real" one from the silvered surface reflection) You can tell if this is the case by looking edge-on at the glass, that will reveal which side has been coated.
I wouldn't touch them unless it has a noticeable impact on picture quality; the coating was probably done with silver nitrate and traditional "mirror" making chemistry. It should be able to stand up to a _very_ gentle wash if it's only surface dirt, but I certainly wouldn't try anything more vigorous without a backup plan; you never know how storage conditions may have affected the coating's bond to the glass.
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