Thats is my guess as well. There is no clear "cutoff point", it is more of period of time unpatched and patched switches are produced at the same time
in the same factory. After a while, the factory will have upgraded all machines to only produce patched switches. It's the same with firmwares in my experience:
That said, buying any Switch in the XAW100800 range in definitely a gamble. Same with serials that are close to known patched serials.
This is true. My local target has all the new Serial: XAW100826*****. I returned mines and ask the rep if i can look at the serials before buying. FG doesnt work on those. Luckily they had the 700s in the back and got one. Both my purchases for the 700s work fine and was purchased mid july.