How do you select the correct mainspring when ordering a replacement? I have a 16sz. Illinois watch made in 1902 that needs one.

I think we need the serial number off the movement first. Then we can determine what model Illinois you have. Then the mainspring number can be determined.

Shoot, I have that serial number!
1623265.

Yours is a Model 5. The mainspring number is 47317.

I see that there are some available (Swiss made) at www.dashto.com

He has them listed as 47317-T.

Thanks Fellows, I have one ordered!