The stamp places do have experts in each field, and will occasionally issue one saying "we decline to render an opinion" some specialties have very few experts, and some items are very hard to be sure what exactly it is.
They take their time, and their error rate is very low. And on more common items, nearly 0. (I'm sure they mess up occasionally, but I've never seen a cert I knew was wrong. )
Coming from a machining and equipment repair background, an error rate of 0 is always the goal.
Some jobs didn't work out, but they generally didn't leave the shop.
some jobs, the downside of being wrong was that someone could die. Those were done with extra care.
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