This is the balance from the first of 3 window banks. The numbers stamped on the metal channel differs but otherwise they appear to have the same dimensions - 13 5/8" long, 9/16" wide. The top and shoe appear to match Series 835.
The spring length differs but I assume that is due to wear.
The window panel they fit in are 7.5 to 8.5 pounds.
The stamp on the metal shows "13 1 B.S.I. RB" with the "B.S.I." tilted 90 degrees.
The other two, which I will post next show "13 2 B.S.I. PT" and "13 3 B.S.I. PV".
Dennis