Hi Miriam, I'm not sure what you mean by "a big old belt on the bottom" of the machine. Do you mean inside the machine, like when you look inside it from the bottom? If so, the Singer 237 does not. The interior is entirely made from metal parts, the only belt is the one connected to the motor.
The Singer 206s, 306s and 319s all have fiber timing belts inside them, if that's what you mean; but the 237 is a different animal.
- Rain