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Cristian is right. You can re-grind it to any focal length you like but I tend to agree with Jason. There are probably a few people out there looking for an F3.5. If the figure happens to be good then it would be a shame to destroy it.
Could you strip the coating and have someone test it for you? You can remove the coating with ferric chloride (the stuff you etch circuit boards with). You soak the mirror coating in the chemical overnight.
A simple ronchi test will tell you if there are any gross errors (rolled edge etc).
Rod.
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