I have used gauge blocks in the past to measure groove width before, the issue is that this bore was only .625 in diameter and the groove width had to be 0.188 by .875 diameter it was recessed back into the part 1.125". I could have used a depth gauge from both ends skewing it out of square to catch an edge with the anvil then subtracting to get width, but then I would have to take the part out of the chuck every time and indicate it every time, not to mention (although minimal) the cosine error that I would add by using the tool out of square. For this application (custom hydraulic cylinder) the groove mic would be perfectly fine, it was just a seal bore so there is tolerance.
I originally had hoped to find a Starrett, however I found the Mitutoyo even nicer, longer handle, a little bit larger anvil, more robust feeling to it, and there is a move-able colalr that can be tightened in position to reference a shoulder or otherwise. Also, I know it is sacreligious here, but I am trying to get away from Starrett. Ever since this big "youtube" push to be a machinist, prices are getting out of hand and the sellers are loving it. I get it, nostalgia, and a recognizable name. I think the resurgence is great but in a few years when there is a new "trend" will all these tools show up again or will they disappear into the never-ever-never-venture regions of garages and basements. I really hope not, but time will tell.