I don't think we can help you on that one. You can contact HP directly to see if they have a record of it.
My ASUS AC66U router has a list of the wired and wireless client devices that are allowed to connect to it. Everything that connected to the router stays in the list until it is refreshed. The MAC address for each device is present.
If you have an advanced home wireless router, finding the MAC address could be as simple as signing into the router's web GUI.