Just took a look at a couple of devices, an MX100, MX84 and MX64 and they all show that the MAC listed in the local webpage under the Security Appliance details is the LAN MAC. Same with the MAC listed in the dashboard under Security Appliance --> Appliance Status
Also, the WAN1 MAC is one higher than the LAN MAC and the WAN2 MAC is one higher yet. So:
LAN
xx:xx:xx:bf:38:00
WAN 1
xx:xx:xx:bf:38:01
WAN 2
xx:xx:xx:bf:38:02
and
LAN
xx:xx:xx:8e:42:08
WAN1
xx:xx:xx:8e:42:0d
WAN2
xx:xx:xx:8e:42:0e