The production quantity estimate based on known serial numbers is approximately 1,100, which does make it a rare pistol. I would say in good condition with one mag you are looking around $550-$600 OTD, more if it includes the original box and additional mags.
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My estimate may be a bit rusty, it is hard to track 4056TSW prices when they so rarely go up for sale.
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Please let us know if the serial number is beyond MSE1048 - MSE2076.