Pat,
Are there working hypotheses on who and how the twelfth came to be?
My guess is that given the era, new designers tried novel features, and that GCGC 12 may have been one of those. It looks like an exaggerated thumbprint intended to combine both a vertical and a putting hazard if you missed left, right, or long.
Reversing my point above, I could see that as an Emmet feature. I think of Emmet as following his own compass.
Tom,
Widen the net: have you ever seen a feature like that anywhere?
Thanks, Dave