Since Irish lad is in HKG and started in CAI I reckon 20 would be a good guess, and the first two were CAI-LHR-HKG.

Am I correct in assuming that this ticket was issued by BA in CAI, not Emeco/AA?

My recent experience with Emeco is that, after a steep learning curve 3 years ago, they are now pretty good with these tickets, and always print out and staple the full fare calculation to every coupon if it can't be fitted on the printed/handwritten coupon itself. That makes reissues a lot easier.