According to http://varietyvista.com/San Francisco Mintmark Styles.htm the only mm was http://varietyvista.com/Mintmark Styles/1952SMMS008b.jpg but over here http://www.coppercoins.com/lincoln/diestate.php?date=1969&die_id=1969s1mm004&die_state=mds they show a different one for the year and don't even note that it is in error. http://www.coppercoins.com/lincoln/1969/1969s1mm004m.jpg What gives?
Believe it or not, it is actually the same type of mintmark. It's appearance can differ due to depth of punch, die abrasion, and a number of other factors. Before you try to call out some people that have been attributing die varieties for decades (the coppercoins.com attributors), maybe it's best to do a little more research.
They do note on CC that they are S/S RPM(s), which gives it a difference appearance from the normal non-RPM mintmark. I believe once you adjust for the RPM effect they are the same mm.
I took it that your comment of "they show a different one for the year and don't even note that it is in error" meant that you knew something that they didn't. If that wasn't your intention, my apologies.
That may be it, but I don't understand how additional punching can make the serif/ball/tail smaller/thinner?
The depth of the punch can affect how much metal flows upwards when the coin is struck, thus also affecting how much detail you see in the MM. As stated before, it also may just be due to die wear or die abrasion.
