If ethernet bridging with indoor ap's isn't working in 7.3 or 7.4 code, upgrading to 184.108.40.206 will very likely correct the issue
As tested and verified, 220.127.116.11 and 7.6.110, have all of the fixes for the various indoor ap mesh ethernet bridging bugs.
Outdoor AP's (1520, 1550) are not affected by these bugs.
Indoor AP Mesh Ethernet Bridging Bugs
CSCub67462 ethernet bridged (Vlan Tagging) broken in 7.3 CSCud41334 MAP Ethernet bridged client does not work
Vlan Transparent Native Vlan Forwarding Behavior
With Mesh Ethernet Bridging Vlan transparent enabled, historically (5.x-7.0 code), the Rap’s native vlan would be forwarded out of Map ethernet ports. Assuming the Rap’s native vlan is 1, vlan 1 would be usable for management across the mesh sector, including the Map’s connected switch.
In 7.2-7.4 code and later, this behavior has changed. With Vlan transparent enabled, the Rap’s native vlan is no longer forwarded via Map’s ethernet interfaces.
testing reference: (in progress of update)
This behavior reverts in 18.104.22.168 (native vlan *is* forwarded out of map's ports with vlan transparent enabled) due to CSCub63054.
Other Mesh Ethernet Bridging related bugs to be aware of
CSCue32755 Wireless client not able to associate to map with ethenet bridged client CSCub63054 MESH VLAN Transparent not have same behavior in 7.0 in 7.2 builds CSCud74948 Document Mesh Vlan Transparent Native Vlan Handling Change in 7.2+
Spanning tree related: CSCui60915 Mesh APs causing mac flapping and loop on the switch CSCuc03576 ARP problems with MAPs