Quite frankly this sounds a bit weird – even for the 70s.  It’s a strange combination of a prawn cocktail and grapefruit.  I used to make a grapefruit appetiser by halving a grapefruit, segmenting it, pouring in sweet sherry, sprinkling on brown sugar and then grilling the lot – it tasted quite good and was quick and easy to make.  It was also pretty cheap (a definite consideration back then).  I also used to make prawn cocktails, but only for very special dinners as they stretched the budget  quite a bit, but I never combined the two and I don’t think I’ll start now.