Wheatstone bridge based upon relative resistance, if I recall high school physics. The OM series relies upon 1.35v mercury oxide constant output cell for accuracy, so my guess is it is a series circuit, not a Wheatstone bridge.
The galvanometer without power would rest near the bottom of the screen. Only with power can you make it move to the center position. In a bridge circuit you would need a galvanomter that would rest at the center so I think it has the series circuit.
OM-1/1N uses cds cells in parallel with dividing network of resistors. Meter accuracy relies on 1.35V coming in as with schotkey diode 1N-6263 and 1.55V silver oxide battery (357, 303, SR44W, G-13, MS76, KS76). John, www.zuiko.com