Geoff,
Congratulations again on another first for Po Toi and Hong Kong, and another quality seabird.
Many boobies and gannets often have confusing immature plumages. and care is needed. However, in this case, fully agree with your ID that its an immature Masked Booby for the reasons you have given. The clear cut relatively broad white collar seperates from all other dark-headed immature boobies [ie Brown, Red-footed, Blue-footed and even Nazca, recently split from Masked in the Galapagos, which can occasionally show a white collar but not as broad or clear cut].
Seperation from Brown is also confirmed by the white upper breast whereas Brown has the dark from the head extending down to the lower breast. The white underwing with narrow dark line extending from carpal to near the axillaries is also just right for Masked, and the white on the scapulars indicates changing to white adult back plumage, not brown.
Red-footed generally has a paler head than back, and darker underparts, without the clear cut back demarcation between dark head and breast. Also the underwing is usually darker and messier.
Blue-footed has a white nape patch but this is not connected to the white breast to form a collar. Blue-footed also has white upper tail coverts, whereas your bird appears all dark above.
On range, Masked is known to breed in the Senkaku/Diaoyutai Islands [with Brown] and is regular in the East China Sea whereas Blue-footed and Nazca are only known from the opposite side of the Pacific. Note Red-footed used to breed closer at the Paracels in the South China Sea but is ruled out above.
Lastly, your bird appears to be in wing moult [inner primaries] and back which fits post juvenile moult of last summer's fledglings. This would make its age first winter heading for first summer. Masked Boobies take 3 years to gain adult plumage.
