Sea-Gull, back in 1963, was just the Tianjin Watch Factory (est. 1955). They bought the rights and production machinery for the Venus 17x chronograph movements from Venus, and created some prototypes for the D304 project. By the time the "1963" chronographs were produced for the PLAAF, the movement had been refined, and now had 19 jewels instead of the original 17. The movement was named the ST3. The "Sea-Gull" brand was created in 1974, and replaced the previous "Dong Feng" brand name (1966-1974).

In 2012, Sea-Gull brought back this chronograph movement, increased the beat speed to a more modern 21,600 vph, added a couple of more jewels (21), and renamed it the ST19 series. The original Venus series had come in different variants. The Venus 175 was the 2-sub-dial variant in the 1963/D304 watches, now revised to become the ST1901. The ST19 series also includes other Venus 17x variants with additional dials and complications.

All the ST1901 movements found in current 1963 chronographs have 21 jewels, but some state "19 Zuan" on the dial as a nostalgic throwback to the original ST3 powered watches from the 1960's.

Sea-Gull sells high quality ST1901 movements to third party manufacturers, but manufacturing cleanliness and quality standards can differ across brands. The Sea-Gull branded watches are assembled with greater care, and in cleaner conditions, than many of the other brands out there.

If you want to have your watch adjusted for more precise timekeeping, the ST1901 has the option of a "Swan neck" feature, which simplifies the process by adding an easier way to adjust the timing. This is especially helpful for DIY adjustments, but can add $20 to $40 to the overall cost.

https://www.watchuseek.com/threads/sugess-zi-zuan-sea-gull-are-they-all-the-same-plus-custom-dials.5340115/