Don't feel bad. You didn't pay too much, you just bought too soon.
And besides, look at the asking prices on all the current production junk guns - even before the current stupidity going on. You bought a superior firearm for not much more than many lesser guns.
And then there is the never say never with s&w. There were "standard" production 3rd gen pistols made with forward slide serrations, from the factory. Info on them is extremely hard to come by. Some were small batches of Salesmans samples made to show LE Agencies and used for T&E by some of them.
Others - unverified at this time - appear to have been sent to the Early Performance Center, 1991 or 1992, for customization by their owners. That service was offered by the Performance Center for a short period of time after their founding.
There have been 4516s, 4506s and 4566s shown here on this board with factory forward slide serrations. I own a black 4566 with them. It was one of the sample guns.
Enjoy that nice 5906. I look forward to your range report! Regards 18DAI