According to the data collectors at Needlebar, I own a 115 with the earliest known serial number (According to Chris, who has been collecting data for 30 years). If you do a search for my machine's serial, it comes up as a model 27, but Chris at NB tells me that Singer was known to stop production of one model to make a short run of another. Mine comes up as having been made in 1911, but the first recorded (by Singer) model 115 machines were made in 1912.
I'm pretty sure the differences between the 15-30 and the 115 are probably mechanical, since the 15-30 was being produced earlier...I can't recall for sure but think the 15-30 I saw was a 1901? I'll check some more tonight when I get a chance...or maybe one of the others can tell us what the differences are?