He first appears on the 1906 Jurors list:
|c||Kastmann||Karl Adolf August||Assistant||Siemssen & Co.||Queen's Buildings|
He's not shown on the lists for 1907-1910, but appears again on the 1911 list with exactly the same details, then disappears again.
The next appearance is on the Jurors list for 1926, which shows a Karl Kastmann working as an assistant at John Manners & Co. He stays on the lists until the 1938 list, which shows him as a Merchant, still with John Manners & Co.
He isn't mentioned in the 1939 list, due to the declaration of war between Britain and Germany. The Gazette entry 808 for 1939 includes Mr Kastmann in a list titled: "IN THE MATTER OF THE ALIEN ENEMIES (WINDING UP) ORDINANCE, No. 28 of 1914.", so it looks as though he was interned as an enemy alien. However, next year there's a Gazette entry 183 that says the appointment of the liquidator was revoked, but only for Mr Kastmann, not for the other people on the 1939 list.
Brian Edgar adds that:
(Susanna Hoe, The private lIfe of Old Hong Kong, 276)