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Spoiler Alert: FOIS sapphire crystal can be exchanged with hesalite (plastic) crystal without any modifications at all to the watch. My FOIS proves it.
I purchased a FOIS Speedmaster with an original Omega 57 3594.50 dial exchanged. I loved the FOIS fit on my 7" wrist. I found the FOIS to be nearly perfect. Except for the milky ringed relatively flat sapphire crystal. Many articles suggest switching to a Speedy Professional for the more traditional hesalite crystal. I considered…
