Does the Apple Watch use SHSH ?

Hello,

Today I wonder if the Apple Watch use SHSH like iOS devices do, or any othe signing system.

I own an Apple Watch 1st gen on watchOS 4(.2 I believe) and I found the IPSW for it of watchOS 2.1 which can be paired with my iOS 9.3.4 iPhone SE.

And in theory I could buy iBus for Apple Watch, put my Apple Watch into DFU mode, resore using iTunes and the watchOS 2.1 IPSW file and I am good to go.

Thanks for your help.