Two different shaped crystals for Monster?
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One of the forum members kindly told me that the crystal of my OM could be a replacement. I got my OM on eBay and is supposed to be a brand new watch. But, I found some scratches on the bezel. It all makes sense now. Also, besides the difference of the shape, the crystal of my BM looks much higher quality than the one on my OM. Maybe, I should check the movement too. At least, it has been working great so far.
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It does make the watch look different.
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When the monster came out, the crystal had a more pronounced dome then it does now. The original dome was only used for a year or two.
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Thanks for the useful information. That was my initial guess since mine was manufactured in 2001. Regards.