Luther on the atonement

I have heard it argued by some reformed Christians, including Timothy George, that Luther taught limited atonement. Clearly, in his later writings he teaches a universal atonement. However, in his lectures on Romans in 1515 he seems to teach that the atonement was only for the elect. While I had previously thought this was the only time Luther made such a statement, I have found something in an early sermon which teaches something similar. In an exposition of Hebrews chapter 1 Luther states that Christ "has poured out his love for us and made purification for our sins. The apostle says "our," "our sins;" not his own sin, not the sins of unbelievers. Purification is not for, and cannot profit, him who does not believe." (Complete Sermons of Martin Luther volume 3.1 pg. 180) Unfortunately, this sermon does not have a date in the volume, though it his clear in his discussion of the two natures of Christ that he has not yet engaged the issue of the communication of attributes of the divine to the human nature. Thus, it is one of his earlier sermons. It seems that Luther did hold to a limited atonement at the beginning of his reformational career. He did eventually abandon this and clearly teach a universal atonement while still retaining the doctrine of predestination.

With this in mind, I am curious as to the reasons Luther abandoned this older teaching. I wonder if the writing of St. Prosper "The Call of All Nations" had an influence in changing his view, since it is a work he refers to several times in his writings and letters. Prosper himself taught limited atonement and abandoned it for the paradox that God predestines specific men unto salvation yet also gives universal grace to all mankind. Luther would not have known this as he thought the work to have been written by Ambrose, however.

Has anybody else run across these kinds of quotes in Luther's older works, or know when this sermon was written?