To add to the good information Candew posted:
The Forum name on the movement probably means that the Illinois Watch Company built the movement as a private label watch for the firm that placed it in the Elgin Giant Watch Case Co. case.
You can see a catalog description of the factory No. 405 grade, along with a picture and where it fit in 's line of 12-size movements, in a
1921 Price List. This was a very, very popular, medium grade movement of which over 390,000 were made. Only a small, indeterminate percentage were marked "Forum."
Please feel free to ask about anything that isn't clear to you.
Good luck,