I made a mistake and thought about that gauge too long. Not possible to make a quick and dirty gauge now. Thinking is dangerous!
Found a length of seasoned gray iron, milled it square and milled the ends also. Surface ground it top and bottom and milled the 5/8 channel. I was going to mill a groove to clear the bolt guide but that would have required a stand under the gauge to clear the hanging bolt handle. Decided to mill a stop to support the bolt handle instead. This keeps the bolt from camming out of the channel from the weight of the handle.
Need to make an indicator holder and a gauge for zeroing the indicator and it will be finished. Regular work keeps getting in the way, but another week or two and it should be finished.
BR, can you line up some more bolts to check and note the markings?

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