You got a very pretty machine - I love the Tiffany decal set.
It's most likely a 15-30, but may also be a 115. The differences are only visible from below. The 115 is a rotating hook, whereas the 15 is an oscillating hook.
To see the differences between model 15 and 115, check out these threads:
http://www.quiltingboard.com/vintage...e-t216990.html
http://www.quiltingboard.com/vintage...l-t235346.html