M.Ted,

Actually, the thumbed dominant seventh chord isn't really strange at all. Follow this and you'll see.

Make a standard first position full C7 chord. By that I mean a three-fingered C mjajor chord using the little finger to note Bb on the G string and making sure your ring finger is covering both the bass strings so you could play a three note alternating bass. Never mind if you don't normally make it that way, just go along for now.
Now that we have the standard C7, we want to move the G from the bass E string to the trebble E string. In order to do that we need to move the little finger over to the trebble E string and move our ring (3rd) finger over to the G string to fill in for the missing pinkey.
Now we only have a four-string C7 chord so we fill it out by thumbing the bass strings at the third fret taking the place of the purloined 3rd finger.

You see, it's really just a standard C7 chord fingered so that it becomes mobile.

You can play with a lot of chords this way. If you learn the chord spellings, you can find a chord that sounds right wherever you are on the neck.

- Mark