Of all the pictures I have found, the roller I think I need is a G 3121 - Sliding Window Roller Assembly
It measures about 3/4" long by 7/16" high by 1/4" wide, and the roller appears to be about 5/16".
I have added a picture of the roller, and a picture of the window from which it came. The rollers (2) are on the bottom of the window that slides from right to left.
I need 12 of them so I hope you can help.
Thanks, Dave