Here's a fun fact, the ST-19 is not a copy of the Venus 175 per se... it technically IS the Venus 175 because they bought the tooling to make the Venus 175 in the early 60's. There are two main differences between the ST-19 and the Venus 175... the incabloc style shock absorber and the beat rate, which was increased from 18,000BPH to 21,600BPH.
The movement comes in 6 variants:
ST1901
2 register,
small seconds at 9,
30 min counter at 3
(Ex Venus 175 - parts compatible, Venus parts can be used for repair)
ST1902
3 register,
small seconds at 9,
30 min counter at 3
12 h register at 6 (non chrono!),
The 12h register's hand is directly geared to the main hour hand - bloody useless. See the pictures of the "Schaferwatches" 3-register chrono, the 12h in the register is always in the same position as the main hour hand.
ST1903
3 register,
small seconds at 9,
30 min counter at 3
24 h register at 6 (non chrono again),
The 24h indicator is directly synchronized with main hour hand, either used as 24h indicator or with the hand replaced by a day/night disk as a faux moon phase - day night indicator.
New variants 2007
ST1905
2 register,
small seconds at 9,
30 min counter at 3
de-central power reserve at 4:30
(Plazeon chronos)
ST1907
2 register,
small seconds at 9,
30 min counter at 3
central power reserve indicator
ST1908
4 register,
small seconds at 9,
30 min counter at 3
date at 12
day/night window at 6
No info about the "missing" denominations ST1904 and ST 1906.
Seagull is developing or considering:
A true 3-register chrono with a 12h chronograph counter at 6, prototypes have been built, this might be one of the missing denominations.
A GMT variant of the ST1903, where the 24h register can be set independently - only in consideration.
The addition of an automatic winding mechanism to this movement (allegedly prototype status).