That rad is different to the one shown in Fontyyy's post but it is a HRC NSR one.
Basic rule of thumb is flat = '88 (thats the year 1988 not 88mm Fontyyy) and curved = '89. Both MC18
The '21 used a 3 hose system as did some of the '28 HRC kits.
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If I have to take the carbs off once more...