I re-measured the bottom shoe. It looks like it is 1 1/8" long, so the balance may actually be a Series 390. The length question still remains, however.
Also need one top plastic guide. looks like an 18-110. Dimensions match. Reverse side view is slightly different. See photos attached.
K window - balance stamped 1820 B. Measures 18 1/2" long for the metal channel piece. Appears to have the same top and bottom shoes. The top guides are intact for this window.

