Skipjack (TicWatch C2/C2+) and Tunny (TicWatch S2/E2) are a series of compact, feature complete watches.
Skipjack are round-screened OLED watches which all come with GPS, compass and easily accessible USB. Despite their shared innards, they have significant external differences. Unlike Catfish, these have no speaker.
C2 and C2+
Both of these are compact watches with steel cases. Both the C2 and C2+ come in 18 and 20mm strap variants. The straps are easily replaceable. The C2+ seems to just be a ram upgrade to 1GB over the standard C2's 512mb. This doesn't have a measurable performance impact on AsteroidOS.
The S2 trades the metal case of the C2 for a larger plastic case, with a rugged look. Unlike the C2, there is also only one button.
The E2 also has a plastic case, but this is just plain and round. Like the S2, there is only one button.
C2 and C2+
These are held together with glue. Opening will compromise water resistance. Parts are not generally available
S2 and E2
These are 4 torx screws on the back, but it is not known whether water is kept out with a glue layer (such as on TicWatch Pro 3) or a rubber o-ring (such as on previous Mobvoi models). Parts are not generally available.
Miscellaneous hardware docs
Skipjack: The upper key is the power key. Tunny: The only key is the power key.
Manually getting to fastboot
Skipjack: Power down the watch. Keep holding both buttons during the boot process until the fastboot menu appears. Tunny: After the watch stops vibrating on startup, immediately touch the top left & bottom right edges of the screen. Tapping repeatedly after pressing the central power key may be useful.