I've had some doubts about this coin, but I compared it to the few auction examples I found, and couldn't find any inconsistencies.
(
This one, in particular, looks like it might even be an obverse die match.)
I apologize for photo quality; either the coin has darkened considerably since July 2017 (when the photos below were taken), or I lucked out into a particularly bright sunny day back then, but one way or another, my later photo attempts sucked at contrast.
Ironically enough, this only cost me $7 (or so), and (arguably) wasn't even a deliberate cherrypick...

Elbing, Swedish occupation of, solidus, 1657.
Obverse: CAROL GVST D G R, crowned CG monogram within inner circle
Reverse: SOLIDVS PRVSSIAE 57, arms of Elbing in cartouche within inner circle
KM# 65 (listed as "rare" - no price given),
Numista 96629 (rarity 97, which means that I'm the only member who has it), Kopicki 9655 (R7, estimated 4-6 examples known)
It's a bit hilarious how I picked this up. I went through the $7 per coin (...well, $7-ish, I forgot the exact figure) bargain bin, this coin caught my eye, I looked at the legends - "Carl Gustav of Prussia? Hey, wait, that can't be right..."
So I bought it as an interesting puzzle to figure out, entirely unaware of the rarity.
For comparison,
this mildly misattributed example sold for about $150 in 2017;
this one (the Numista photo) sold for about $500 in 2012. Both seem to be in better condition than my coin, admittedly.
And for the record, yes, I cross-checked
again with a PDF copy of Kopicki, and this is 9655 and R7.
...I vaguely recall having been told that this coin was supposedly sent to the Volgina stamp shop (where I bought it) by Vasily, then of Hobby Market.
If so, that would totally fit what I know of Vasily's shop... that place had
a lot of scarce-to-rare (mostly ancient) coins, and the owner could hardly care less about most of them. It was (and probably still is) heaven for the wannabe cherrypicker.