Pete
I have never seen this light before on any classic and our maintenance training manual makes no reference to it. Maybe it was installed on –200’s and migrated across if your airline, or the previous owners, operated both types. My guess is it shouldn’t be in your QRH. What do your engineers / fleet managers say? I presume you still have the SPEED BRAKE DO NOT ARM and SPEED BRAKE ARMED lights and this doesn’t replace either of those. Let us know what you find out.
S & L