As a variety collector/hunter, personally I'd open it. Last batch of 65 SMS sets I opened had two really cool die clashes (one dime and one cent.)
I don't know if the difference between SMS and regular coins can be detected UNLESS they are in their original packaging. I don't collect 20th century US coins so I don't know the difference.
Haha I totally forgot. Here ya go. Coins of course but the value can drop once opened. Packaging is part of the valuecwith these moderns.
right now it could be a MS70 early die state, fully struck rainbow toners set, if you open it it might be a MS61 LDS weak struck set with black toning. Sometimes what you don't know is better than what's known. It's a mystery what might be inside!
There are ways to open envelopes using steam. I know so because I saw it done on an episode of the Brady Bunch. I think it was Alice that did it.