According to
http://www.singerco.com/support/serial_numbers.html, your FW's serial number was released for production September 16, 1946. Also, it was manufactured in the Elizabethport, NJ factory, so I'm not sure where the conversion comes in, unless someone took it to England and had it converted to 220, then returned to the US and had it converted back to 110.
Thank you so much for this info! The machine was given to me when I lived in England and now it lives here in Ireland! I suppose someone took it over from the Us to the UK - but I wonder who and why???????........