Hi,
Welcome to the QB. Your Grand Rotary was made by National SMCo in what I would guesstimate the 30's or 40's. Link to National pics in the Needlebar site:
http://needlebar.org/cm/index.php?cat=91
If you plan on sewing with your National Rotary you'll need Eldredge Rotary needles or Boye 21 needles and National / Eldredge round bobbins. Eldredge is a popular National brand name. With any luck you'll have needles and bobbins in the cabinet.
There is a National SMCo Yahoo group that might be able to accurately date your Grand and also have more in depth info.
https://groups.yahoo.com/neo/groups/nsmco/info
This should be a very nice working treadle with little work. Good Luck and Enjoy!
Jon