John,
I think your photos show both taxa of Asian House Martin that occur in HK.
Note that all your birds are first-winter birds based on the white tip to the tetials.
The two Asian House Martin taxa that occur (nigrimentalis from se China and Taiwan) and nominate dasypus (ne China, Korea, eastern Russia)can generally be separated on size, structure and plumage. The former is much more sullied below and on the rump and is smaller which a shorter more shallowly forked tail (as per the 2nd and 3rd images in your second post). Nominate dasypus is much cleaner below and on the rump, is larger and has a longer and more deeply forked tail (4th and 5th images).
