too many devices, confusing names

I am about to add a third device to my photography plan -- but when I go to "manage plan" it shows three devices already but I believe I only have two devices.  I know what one of the three is (MacBook Air), but the "device names" for the other two makes no sense to me.  One of them should be my "old" iMac, which I still have and need to use.  The weird names are "[my name]'s Desk on Mac" and the other "[my name]?s desk onOSX"

How do I discern the the correct name to go with my old iMac so that I can delete the third (spurious?) one.

how would I delete that one?

Where do these names come from anyway?

(then I need to start the process to add a new iMac.  I know I will have to deactivate something, but would like this cleared up so I don't screw things up now or in the future.)

thanks