The bolt should have a grade on it. 8.8, 10.9 for metric or 2,5, or 8 witch are the same as the hash marks. From what I have seen alot of stainless quality bolts do vary. Anything from 18-8 to A-2 and diamonds with hashes but a quality bolt should have markings on the head instead of its size. I have seen bolts at ace hardware that had the size on it with thread count. A stainless bolt stretching is strange im thinking it has some aluminum alloy in it to give it the stainless look. A decent stainless bolt has around 40,000 tensil strength. Now if your looking for a bolt that you can polish and not rust iconel is a good bolt that is usally splined and has threaded open head that allows to screw a transducer into to measure its length while its being stretched. The machine to measure its value is expensive though.