I know some Win 100's were rebarreled with win 88 barrels in .358 Win and have seen them for sale in online gun auctions were they stated they were rebarreled. I do not believe changing the caliber to .358 Win would be difficult. Look at the AR action and numerous calibers now being used. I currently have a H&R 700 auto in .22 mag and have been considering converting it to 5.7X28. The only thing stopping me is the cost of haveing a new bolt made by EDM to move the firing pin to the center. I'm wieghing the price as opposed to buying a new gun. The gun in your case could have been a special order. In the link I provided, http://www.gunauction.com/buy/109143...n-.358-caliber
; look closely at the "5" in the .358. It is not the same as the 3 and 8 stamping which are heavier stampings than the 5. Obivousliy, it is not a Winchester 100 that came off their production line. However, that does not mean it wasn't made by Winchester as a speical order.