It appears to be NGK r6918b-8. Doing a little searching I found the spark plug above listed as: "NGK special spark plug for (1996-2000) RM 125." This same spark plug is listed in the 2003 RM 125 Service Manual as the correct spark plug for that engine.
At least one spark plug manufacturer (NGK) recommends a gap of .028" for a 1983 Suzuki GS550 (this is for the D9EA spark plug). Interesting to note, a 1982 Suzuki GS550 uses a different plug (B8ES), and the gap for that one is .032". So if you have a 1983 model, that gap should be .028".