On SRX5600 and SRX5800 devices, the out-of-band management interface fpx0 has a non-Juniper MAC address.
Juniper devices normally use the Juniper MAC addresses on their interfaces. However there is an exception with the out-of-band management interface fxp0 on the SRX5600 and SRX5800 platforms, where the MAC address is not from a Juniper MAC address pool.
root> show interfaces fxp0 | match "Hardware address"
Current address: 00:a0:a5:68:f3:76, Hardware address: 00:a0:a5:68:f3:76
MAC OUI lookup will show that this MAC address is from the pool of TEKNOR MICROSYSTEMS, INC.
The non-Juniper MAC addresses used are derived from the MAC address pool of the manufacturer of the routing engine for SRX5600 and SRX5800 devices. These MAC addresses are still unique and there is no concern of MAC address conflict in the field.