I'll have check my examples of the f-20's when I get home next week, however at quick glance I see the 2 varieties as I referenced them. A high last 1 in the date and a low last 1 in the date. If all the low last 1's are to the gap and all the high last 1's are to the tooth then we have a very simple method of differentiating between them. Not sure if this is significant or true as I do not have the coins in front of me, but I should have at least 2 or 3 of each variety to check....
I can't speak on the f-28's as I only have the one in my collection, but if I can find images of the 2 that I sold in the past I'll try to compare them....