I think a resister won't work because it doesn't drop the voltage the same in different current loads. On a CdS meter, like the SRT has, the battery puts out 1.35v but the amount moving through the circiut is altered by the resistance of the CdS cells, which changes based on how much light hits the cells, so a resister placed in the circuit won't always gve the required .2v drop, which must always be the same drop no matter how much current comes through it from the CdS cells. A diode does produce the required voltage drop no matter how much current the CdS cells pass through.