I just noticed that mac address of a serial port is not there in a 2911 router in PT. also while using eui-64 the serial picks up the mac of some a gigabit or fast ethernet interface.
Serial ports don't have or need MAC addresses. It's as simple as that.
MAC Addresses are specific to Ethernet interfaces - serial interfaces are point-to-point and use a different Layer 2 protocol (PPP, HDLC, FR etc.) for communication.
EUI-64 is most likely deriving it's address from another source on your router.