What is the series of model number mu6e2ll/a Apple Watch?

That is not an Watch model number. Watch model numbers start with an A followed by 4 numbers.

A search of the internet with that number identifies a similar number associated with the Series 4 Watch on the bhphotovideo.com website as a manufacturer's number.

M1 Mac mini, macOS 13.3.1;  Watch, series 7 GPS/cellular, watchOS 9.4; iPad Pro 2nd Gen, iPadOS 16.4.1; iPhone 12 Pro, iOS 16.4.1; TV 4th Gen, tvOS 16.4.1; HomePods, audioOS 16.4.1 and Airport Express w/ Airplay 2

It is also a GPS only model, meaning it lacks a cellular radio.

That number corresponds to a Space Gray model. Dah•veed's information is otherwise correct.

(238571)