You can see the line number above the hold, and this is different to the actual MSN for the aircraft which (I imagine) run sequentially across all product lines (73, 74, 77 and 78)
Well, if it was painted 19 times then....
Possibly just a case of Boeing, with better visibility of the relative completion states, production and delivery schedules for these MSN's than us mere mortals just finishing off the livery enough for each to take it's sequence of test & pre-delivery...
doesn’t quite fit.