re: St Louis Cut Dry Ribs Memphis Style?

Couple things:
275 isn't a bad temp, especially for baby backs. I know you cooked StL, but try some baby's next time.
Don't soak your wood. Some of that "smoke" is just a little steam. You'll get a better, cleaner burn with the dry wood.
Let them go for a few hours, spray with a lite mist of water every hour or so, look for the rendered fat to surface, then start checking for doneness.
A few doneness tests:
•bone pull (xylophone test from above). Does it feel like you can almost pull the ribs apart by hand, without them being too tough, or falling apart. This requires some experience over time, and you'll learn what to look for, once you decide the texture you want to eat.
•bend test. Very popular, but a few things can dictate it's flexibility. Like how far into the cook you are, fat content of the rack, dry vs wet heat on surface texture, thickness of the rack, etc.
•internal temperature. It's easier to get a true IT on a bigger piece of meat, than it is for a rib. Instant read thermometers read at the very tip, so you'd have to know if you're dead center of the meat, and exactly between bones.
•probe/toothpick test. I'd rather this over an IT. How does the meat grab? Tight, slight grab, or like butter? Again, depends on your eating preference.