There are a few differences in the 15's and the 115's. The bobbin for the 115 is a 20U [ATTACH=CONFIG]363731[/ATTACH](still available)
The 115 was considered to be the "Top of the Line" of the time. I have the machine with the earliest recorded serial number for this model, according to a person on Needlebar that has collected serial numbers (all models) for more than 30 years. I saw somewhere that Singer only made about 300K of the 115's, altho I think that number may have been for only one year, or totally off....I've never added up the 115's on the serial number lists.
There are no differences for the attachments...what fits your 15-91 will fit your 115. Standard short shank attachments.
My 115 has a boss under the handwheel on the pillar, so there shouldn't be a problem if you want to handcrank your machine.