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Jo,
I believe the "coating" is just an AR coating with some IR rejection.
There's only one (thick) filter in the older Canon's and as far as I know (based on the filter glass specs) the glass is providing the "colour balance" correction which has the dramatic effect on the red sensitivity.
If you can obtain measurable Ha intensities and compare this may be easily resolved.
A slit spectroscope and the solar spectrum is one of the easiest ways of verification.
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