Did you check the size ?? P. Schenk was mainly a publisher and printmaker although the family did a few original maps,, this was not one of them on the list. They did use plates that were done by others. Looked at one of the early maps like this and it is considerably larger than this one !! 22 1/2 X 24 1/2 I think,, can`t remember exactly !! Are you sure that your map isn`t a 16X20 counting the part covered by the frame ?? That size is the single most common size for stock frames so most all printmakers use that size or a few of the other stock sizes rather than having frames made for an odd size !! Frames like this are made in bulk !! I also looked at a few similar to this on ebay,, thgey had the same centerfold !! That makes me think it may be from a book of some type !! Schenk did do some world atlas,, small possibility it came from one of these !!
Just my opinion,, have it checked !!
