Ok...I learned something "new to me" over on Needlebar tonight!
My 115 has a serial number that Singer lists as a model 27. Chrys had told me earlier that this serial number is the earliest serial for a 115 on the records. (The ones that Chrys has been gathering for 30 years). So then I had to ask if the entire run of serial numbers would have been 115 machines or if my machine having that serial was a quirk?
Chrys responded: "I have several 27 sphinx machines in my database in that batch of serial numbers. There was probably a small batch within that range of numbers that were 115s. I've come across this before. e.g. There are batches of Featherweights within larger batches of other models that never got recorded."
Chrys impresses me no end...as does the willingness of the other members of Needlebar to help!