How does a momentary dpdt switch function?
There is no guarantee it will be the center terminal that is common. Just probe it out. It will almost surely be symmetric on either side, but it may be one of the end terminals that is the common for each side. 

In any case, there are only two 'states' for the switch, so write down what is connected to what when the plunger is pressed and what is connected to what when the plunger is released. If there are 6 connections you need to make a total of 30 continuity tests (5 + 4 + 3 + 2 + 1 for each state) to be completely systematic about it. If you short a few together and have a good guess you can cut it down to just a few tests. 

Edit: Here is a similar looking part, and the middle indeed is the common: