Hi Buford,
I don't know the 690E at all, but it sounds like you're on the right track so far. This pedal that you found - is it a rocker pedal or a single position pedal? If it's a rocker pedal then you have 2 way hydraulics going up front that may have at one time been used to run a tilt bucket or some sort of clap or who knows what. If that's the case then it would be fairly easy to install a thumb. The only thing you'll want to make sure is that there are port relief valves installed in the aux. valve so that your bucket can overpower the thumb and the hydraulic fluid from the thumb cylinder has somewhere to go to, if not, you can bend a cylinder, blow a hose, etc.
If your pedal is just a single position pedal the most likely what you have is a one way circuit up front, usually used to run a hammer. Sometimes they have accumulators connected to the aux lines on the back of the main boom to take some of the percussion out of the hydraulic lines. Also, one line will likely be bigger than the other in a one way system.
Hope this helps somewhat, I'm sure more people will eventually be along that can help you out.