I also took a bunch of pictuers of a 115 and a 15-30 not long ago and posted them here. They are the same on the top side, but very different on the bottom. Mine are both from about the same time frame period (quite early) so the 15 is the older style also.
http://www.quiltingboard.com/vintage...l-t235346.html
I also have a tiffany 115, and I can not find ANY difference on the top side between it and the 15-30.
Any "top side" difference between a 115 and a 15 have to do with age and decal pattern. But the really old 15-30's are quite rare.