Forever (well, since 2010 anyway) I thought that I could tell the difference between the Singer Vibrating shuttle models by noting the position of the bobbin winder. I thought that in the older 27/28 models (the 28 is the 3/4 size) the bobbin winder was always positioned low on the pillar. Now I find that is not the case. The tension release is a more significant clue. The tension release is manual in the older 27/28s (see the little spoon shaped extension?) In the newer 127/128s the tension releases automatically when the foot is raised.
Thanks to the folks on Vintage Sewing Machines Facebook group for teaching me this fact.