serial numbers for Ipads

I have multiple Ipads that are locked due to the fact that we give them to the sales team and after terminated they left there own apple ids and passwords locking them.

The issue I have is How Can I find the Serial Number of the Ipad, and when I push the little i circle icon

SN: numbers come up but on my proof of purchase for my bill it doesnt match that IPAD NONE OF THEM DO.

We have numbers for Serial numbers on my AT&T Bill but on my Ipad is all Letters????