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Yep that happens quite often ... Set the tang on a known flat surface, then flip and check again they can be quite a bit off...

Certain Era's of razors, and certain makes, I check first now...
...

Foolproof ???? Not that I know of :rofl2:

But here are some tips

http://straightrazorpalace.com/workshop/18504-welcome-workshop-how-do-i-where-do-i-what-do-i-answers-here.html

Use that...