My understanding is that the oval window came out in 1929 in order to dress up the otherwise plan-jane Business Coupe. If these "teacup" windows appeared in 1928, that would surprise me. I seem to recall that they were introduced in about May, 1929 with only 1/2 year of production remaining before the 1930 models came out. That limited production time would explain why there are so few examples extant.
Another reason I don't think the oval windows were installed in 1928 soft top coupes is a December, 1928 Business Coupe I studied here in town. It had 16,000 original miles on it and was in no way modified. The features, stampings and engine number dated it to late December, 1928 and the top was devoid of oval windows. I wrote an article for "The Restorer" about this rare survivor. Of course, different assembly plants probably had different introduction times for new features such as these windows, but for what it's worth, I'm pretty sure the oval windows were only installed in 1929.
Marshall