Hint: they were multiple times world champion and had a hard time in winning their home GP: the dutch TT in Assen. Biggest rivals in their era Rolf Biland and Steve Webster
The helmet in the pic does look like Wayne Gardners', I can't remember Gardner running number six (which means little, I can't remember yesterday). It may be one of the Japanese riders, I'll guess at Takazumi Katayama.