One of the reasons William Humes is so enamored with the M7 bayonet is that there are so many variations and more questions like these. There are still many unanswered questions out there. To the point of Paul S's question, it is also possible that his M7 bayonet is a Military Aid Program bayonet that somehow fell into US military stores. Just a possibility.
The reason I said that the Milpar/Colt bayonet has a curved back cut, is that the photos in William Humes' site and also Gary Cunningham's new book, show pictures of them with the rounded back cut. However, if you examine Mr. Humes' photos: http://www.usmilitaryknives.com/humes_m7_02.htm you will notice that although one photo shows an Imperial/Colt and a Milpar/Colt side by side which are identical, in another photo comparison of the tang cut out of the Milpar/Colt VS. no cut outs for the Imperial/Colt, the Milpar/Colt has a 90° back cut. It seems clear that the Milpar/Colt can be found with either style of back cut.
Was the Milpar/Colt a part of a military contract. It has been suggested that Milpar (once they secured the first M7 contract) may have purchased some blades from Imperial to initiate production. As you state Paul S, maybe some of those blades were in different stages of manufacture and did not include the Colt logo. Some also have suggested that Milpar and Colt were working out patent issues, as the Milpar contract got started and used the Colt logo on some of their blades. More questions than answers.
Here's a photo of a Milpar/Colt which has a near 90° back cut, and though not shown, the other side of the blade has a full 90° back cut. Also notice the beveled back of the blade, as pointed out by remman.