So there's no guarantee that this key would work.
I suspect that this person had a Windows 7 or 8 key initially, and it could be perfectly a retail key then, and this is the key that ends with C9XTY. But later this person claimed the free upgrade to Win10, which turns the license to a digital license, hence it would only show generic key last 5.
The problem is - since Sept 2023, Microsoft no longer allows the use of old Win7 or 8 keys (retail or not) to activate Win10. If free upgrade was already claimed prior, then it can only use digital license to transfer. So this is why the old license key would no longer work as Microsoft no longer has a pathway to connect to Win7 or 8 activation server.