Surface bumps and slightly wrong weight, not sure error or fake.

Got a large group of Soviet coins from 1920s to 1950s. I don't know what to think about these two, after I noticed small bumps on their surface, both on reverse and obverse. Some of them got almost perfect circle shape. They are raised above the surface.

The weight for the 20 kop. should be 3.6g while this one is 3.3g (coin shows very minimal wear). The 10 kop. got proper weight. Could it be air trapped inside? They should be 50% silver and they are not rare at all, hard to image anyone fake them.

Would like to hear any explanations regarding the nature of these bumps.

 

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I would have to tentatively agree with the air theory. Look below the 5 on the date of the 10 kopeks. Looks like a burst bubble from the tiny pic on my phone.

 

So what are the technical terms for this to happen? Improper alloy, damaged dies? Is it reducing coin value or considered a mint error and might bring premium?

 

Wonder if it's just enviromental damage .

 

Read the rules James. Keep the political stuff off the boards please.
Taxi, without knowing much about Soviet/Russian minting quality control I couldn't answer with much certainty. I doubt it would bring much of a premium though.

 

This would be my next guess. I had a damaged stone mtn that looked similar.