iphone model A1688

I have a spare iphone which says on the back of the case that it is an A1688 model S which would make it an iphone 6S with a 4.7 inch screen. However the screen measures 5.5 inches which should make it a 6S plus, but in that case the model number should be A1687. What phone have I got is it a knock off, a hybrid or just a mistake at manufacture? By the way I live in the UK if that makes any difference.

Any help would be very welcome as I would dearly love to know. Thanks in advance. fm290.