MCB 6 was probably the first Sea Bee battalion to see real action in the Pacific campaign of WW2, participating in the historic Guadalcanal landing of August 1942. In that respect they are a very noteworthy naval combat unit.
Judging from the design, I would GUESS that the plaque in your possession is the WW2 version of the unit's emblem.
Go here - http://www.mcb6.org/index.html
- for more information. You may also find there parties interested to purchase it.