Probably two top things about which to think: '81 type 2 coins don't look like '79 type 2 coins; If the type 2 "bulbous serif" is present on the '81 coin, it is really easy to discern once you've seen one or two...
the redbook has a good comparison of sba dollars for 79 and 81 type 1 and 11. You need a real good loupe or magnifying glass to see the difference. The s mint mark can be filled or clear. good luck if you can see the s mintmark, look for a bulge on the bottom end of the s on the type 11, also the s is very clear as opposed to an s with two almost round depressions inside the s curves on the type 1.
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Probably two top things about which to think: '81 type 2 coins don't look like '79 type 2 coins;
If the type 2 "bulbous serif" is present on the '81 coin, it is really easy to discern once you've seen one or two...
Jenn