I inverted my new bottle of TBIII (manufactured sometime this year) and it seems to be a little thicker than before. Still not as thick as the 2014 bottle of TBII, but I need the open time so I'm going to hang on to it.
I did do a quick test of the TBIII. I glued a ripped edge of a scrape to the face of another and let it dry overnight. The result of the hammer blow follows...
There a bit of cheat involved here. The scrap that I used had some dry rot on one side. The crack started at the rot and then continued along the grain line. I'm sure the rot weakened the wood somewhat, but it still took a pretty good hit to get it to crack. I probably should have hit it from the other side.