I think all the 15's are oscillating. The 115 is full rotary.
And like others have said, the 15-88,89,90 are the same machine with different "drive" methods. The 15-91 is the same basic machine with the potted motor. So the bobbin area of all four is identical.
Cari - there are also a lot of industrial models using the 15 designation. So some Singer 15's actually are industrial strength!