Lucky you! Nice Tiffany aka Gingerbread decals! Those are my favorite. I couldn't see the "4" before the 66 in the serial number. If it is a 4 then it is probably a 115 - serial number was allocated May 3 1916. "G" serial numbers can be tricky. You can find free downloadable manuals for either the 115 or 15 (and many other Singer models) at
http://www.singerco.com/support/instruction-manuals just put the model number in the search box.
There are lots of threads here about cleaning these old machines including "stickies" at the top of this forum. Be careful with cleaning as there are many products that will destroy the decals. Generally all that is needed is to gently wiping the machine down gently with
sewing machine oil and cotton balls, oiling in proper places, wiping off with soft cloth, check the wiring, belts, and of course, as Mickey2 said) bobbin, bobbin case and needles (which are the standard 15x1 needles) and she should be ready to sew. If you have some specific questions, please ask.
There are several threads here about either machine. There maybe a few about the differences between the 115 and the 15 mainly that the 115 is a rotating bobbin and the 15 is an oscillating bobbin. The differences can be seen on the underside of the machine. They look the same on the top.
Janey - Neat people never make the exciting discoveries I do.