Ask the seller.
If you can’t logon, you’re not going to find out without some serious expense and/or effort. The effort would be put in by someone with some very unusual equipment or some extremely hard to find expertise.
I don’t believe there is a way to obtain that info without login. An alternative way would be to login to IBM’s entitlement login page:
Login or register to login to view your entitled software.
Click on Key Management on the menu on the left.
Your customer id will autofill (if your already registered otherwise have that info ready), just enter your i5 or select from the drop down box.
All the license keys will display which will show the Version/Release of the OS.