There is a lot of confusion between the Imperial or Cambridge open Swan and the Fenton Open Swan.  As I have both swans right now, I thought I'd post a guide to telling the 2 apart. 
The left or blue swan is by Imperial and is the old Cambridge mold.  The pink Swan on the right is a Fenton Swan.  The sizes are:  Imperial is 4" Tall, 3 1/4" Wide,  and 5 1/8" long.  The Fenton is  4" Tall, 2 7/8" Wide,   & 4 7/8" Long.  However, either can vary in size because the glass will change shapes a bit during making.  Still the Imperial is quite a bit larger than the Fenton.
The major differences are shown in the photos.  The Fenton neck is closer to the body.  The Fenton feathers at the beginning of the wing are shaped a bit like leaves.  The Imperial is smooth scallops.  The Tails are different.  The Fenton has separate feathers on the bottom that have lines in them, again like a leaf.  The Imperial/Cambridge has rounded ovals that all fit one against the other.  The Fenton's bottom feathers don't touch. The bottom of the neck on the Fenton is smooth.  The Imperial has feathers.  I apologize these photos are nothing more than thumbnails but eBay won't allow links outside eBay and has reduced these to thumbnails.
Here are the photos to show the differences: