Its a Lisle #13800, GM Ecotec water pump sprocket holder, holds the timing chain and gear in place while removing or installing water pump on Ecotec engines. Holds the water pump sprocket preventing loss of timing or damage to the engine.
Sad thing is, I do not own one, nor have I ever used one. Only reason I know is because there is one hanging on the wall on the Snap-On truck, i've seen it once a week for over a year, so its kinda burned into my mind LOL
I've got a Mac version of this, got 3-4yrs ago. I never had the chance to use it to change a w/p. I did test fit it on a junk engine we were swapping out, bolted up right up good.