Judging by the pictures of the accessories of both machines at searspartsdirect.com:
http://c.searspartsdirect.com/lis_pn...2356-00008.png
http://c.searspartsdirect.com/lis_pn...1871-00008.png
The buttonhole attachment might interchange, but it is hard to tell if the cams will, as the pictorial representation isn't very good.
The feet won't interchange, as they are different shank height(The feet on the 1756 appear to be high or super high shank). The bobbins and needles will work with both machines.
I have come across Kenmore machines with the same first four digits of the model number, but with a 0,1, or 2 at the end and the shank height of the feet are different.