You're correct that it is a commercial bayonet. The seller originally stated that it was one of the original 1200 chevron bayonets but I think most of the regular readers and contributors to this forum knew that wasn't true. What makes it special is the small "S" stamped on the right side of the blade, to the left of the standard markings. $250 is a bit low, I've seen these sell in excess of $500.
It's interesting that there are 166 numbered chevron bayonets and 1034 unnumbered ones and the ones without the numbers are the hardest to find. I've been collecting M9 bayonets since 1987 and have only owned two unnumbered ones but have had as many as eight numbered ones at one time.
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