To find out what you have under the bridge is to look somewhere where the wood has been cut, in this case it's the peg holes. Take a torch and a magnifying glass and look all around the hole. If you see uniform layers all the way down all around the hole, it's plywood. If you see end grain at opposite sides of the hole, there's solid wood. If you see upright stripes all the way down, they're vertical growth rings which means it's quarter sawn solid wood.
If you need to, roll a piece of sandpaper and try to smoothen the hole just to clean any furry stuff. You don't want to enlarge the pin holes!
If you don't have a good enough magnifying glass, use your cell phone camera. Use flash and other light sources and macro mode if applicable. Try to focus as well as humanly possible. The two photos you linked to are blurry and blurrier much due to insufficient light. Use sunlight, mirrors, white paper, torches, flashlights, desk lamps, whatever to get more light into the holes for the sharpest photos possible.
I highly recommend you to look at Jerry Rosa's guitar repairing videos, he's got quite a many of them. This is a short introduction to the subject: