I have one just like it. It's either a 128 or 128K. Mine was 'born' in 1927.
First, go to this website. Scroll down and in the right hand side bar you can enter your name, email, and serial number of the machine. You will get a 160th anniversary certificate. It will open in your browser as a pdf file.
As far as the key is concerned -- call Singer [1-800-4-SINGER (1-800-474-6437), Monday through Friday, 8:00 a.m. to 4:00 p.m. Central Time]. They will send you a key for free. I was surprised that it actually fit the case.
To find out when your machine was manufactured, go to this website:
http://www.singerco.com/support/machine-serial-numbers. Your serial number is located in the front of the machine at the base below the Singer emblem.