Took me almost a minute to realize what was up

Basically, in the space where the mintmark letter should be, there's some rectangle instead that doesn't look like any letter at first glance. (Of course, as any decent collector who knows the realistic possibilities can easily realize, it's a S - just a really worn one.)
...Reminds me of that one time when I found an 1920-E German 50 pfennig in a dealer's junk pile.
If you haven't seen these series, basically all the other mintmarks are normal plain letters, but the E mintmark is relatively fancy.
So I basically found (i.e. picked out of that same junk pile - it was a large one, with mostly German coins) what I thought was the full set (all 15 combinations of the other five mintmarks and dates 1920-1922 - I was then unaware that this type also existed with a less common 1919 date) - and then I see an 1920 with something ridiculous for a mintmark.
I of course buy it, thank the dealer (after buying a few more coins from him - his other junk pile had a bunch of 19th-century "silver" (probably billon) pieces, including a
really worn 1820 Hanover coin I still consider among my favorites) - and then run all over that market asking every dealer what the heck this is!
The most common answer was "it's a F" (I showed them the actual 1920-F, which had a clear plain letter); I think someone said it was a G (I showed him the actual 1920-G, also plain letter).
Then someone took out his catalogue and did find an 1920-E mentioned... marked as "very rare"

so the rest of the market congratulated me for that purchase!
I later checked at Numista; if I remember correctly, there was nothing rare in an 1920-E as such, but there was a rare variety for that date (coincidentally, Numista was also where I found out that I missed 1919). Oh, and to this day I have no idea just
why was that particular one mintmark so fancy
