OK I see it now when I scroll down to the third page here:
https://psref.lenovo.com/syspool/TempFile/cache...
It refers to Some next to each Windows 8.1 and 10 Pro listing, meaning some are for 8.1 and some are for 10. It depends on which came with your PC.
If it came preloaded with Windows 7 I guess it's possible that it works for either 8.1 or 10 but I would check with Lenovo to be sure.
But you said that you had already upgraded to Windows 10, so what's most important is if you remember if it activated when you Upgraded to 10. If so then that Digital License is permanent for the life of the PC, even if you reverted back to 7. It should reactivate with the Digital License it assigned when you Upgraded or contact Microsoft Support phone or chat options here:
https://partner.support.services.microsoft.com/...
When exactly did you upgrade to Windows 10 and for how long? Did you confirm it activated?