Unused Gear
From Splatoon Wiki - SDL Foundation
Note: There are a LOT of unused gear entries inside the Mush data to document, including models?
Splatoon 1 contains a lot of unused gear data inside the Mush data, including ability-named gear, and a lot of test Splatfest-tees.
SUP000 (Splatoon pre-purchase gear)
If you had played the Splatoon 1 Global Testfire and decided to buy the game, you would have these exclusive gear while the Testfire was still available. The gear models would not have been included in the first release. (1.0.0) But, the gear models appeared back in the files since update v80 (1.3.0) and you still couldn't equip the gear.
This is how the Splatoon pre-purchase gear looked like:
SUP001 (Squid Research Lab gear)
The gears are leftover SRL (Squid Research Lab) gears. The devs would use it for events, such as the Global Testfire gameplay. The gear models were not included in the release (1.0.0) but appeared back in the files since update v80 (1.3.0)