Someone said the machine may be inside??? Wouldn't you know if that were true? Why don't you post a picture of it and maybe someone will recognize & identify it for you. I've never seen a cabinet that could hide a machine so well that you can't fond it! I have an old Singer cabinet (not treadle) that I identified after researching Singer machines.
Good luck!

Do you really need to know who made it? Whoever did so, seems to have made it to fit a National Reversew. National ceased production in 1957. It is very unlikely that a machine of any other make will fit in your cabinet, as back then, makers were very careful to make all cases/cabinets brand specific so that their cabs could not be recycled by comsumers to house a new machine of a different brand.

Thanks everyone for the valuable suggestions. While I haven't found the actual sewing machine, just finding more information about the cabinet is wonderful, especially since I can attribute more concrete facts to this cabinet besides it being my great-grandmother's.
I will continue to do more investigation, and once again thank you. Oh, and if anyone has more valuable information on this type of machine or cabinet, please let me know.

There is actually a discussion on this very machine right here on GardenWeb.. http://ths.gardenweb.com/forums/load/sewing/msg050057044786.html
Maybe it will be helpful to you. I would venture to say that your machine manual is probably more valuable than the cabinet--except for it's family history, of course!