What is an SMS "Dot Head" die variety? (great answers get rewarded)

Was reading up on SMS sets and saw someone mention die varieties and "dot heads" - but I've never heard of that before. Is anyone familiar with what that is, which dates/denominations it appears on, and know if it's worth searching for?

If someone can give me a really good explanation, I'll take pictures and share two very special finds from today's SMS set purchases (they're really cool.)

Thanks!

 

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It's a term like "pin head," 'innit? A faint regard for their brilliance.