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The only difference between "Large" and "Small" version of JCV is the number of images (about half in Small varsion).
Are you sure to use the correct position for JCV? (\.System\JCV in internal memory, \Garmin\JCV in external memory)
The Coverage of JCVs matches those of Maps (West or East), this also answer to your question about Poland (i guess Poland lies in East Europe ...)

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There is no specific JCV See Keys, when you download a specific Region with GE, also a specific JCV is dowloaded with coverage limited to that Region.
The code of JCV (name of file) is determined each time by Garmin, no standard rules existing as I know