Based 13th month
I think it was in the sign or be fired 'negotiations' of 1999 when 13th month was incorporated into Based B scale salaries. Based B scales were woefully uncompetitive at the time so it gave the salary scale a pseudo 8.3% leg up, when in fact the total remuneration never changed.
A scalers were not so lucky - they lost 13th month.
Since that time there has been no mention of paying 13month to bases.
Since B scales were inadequate in 1999 it doesn't matter whether they gave them a 8.3% payrise and simultaneously ceased 13th month or just took 13months and divided by 12.
Since the 13th month is guaranteed across the rode at KA, there is no good reason why HKG based CX pilots shouldn't get it too!