S8B5689

So 8 for 88, B for 2nd fortnight of 1988 (3/4 week of January) 5689th bike made in those weeks?

W = a letter, purpose uncertain, possibly indicates a manufacturer or Centurion; X = a number, indicating the calendar year of manufacture; Y = a letter, indicating the fortnight of manufacture (A = wk 1 & 2, B = wk 3 & 4, etc.) ZZZZ = four digit number, probably indicating frame number during fortnight of manufacture.