Helllo Kid,
The magazine (if you will) on the Spencer is mounted in the stock. The spring follower has to be inserted after loading the rounds. The tubes are for retaining the rounds so they can easily be "dumped" in the magazine.
If you search for "Blakeslee Box" you will find the type of carrier used to reload the Spencer. It was a leather cased wooden block which held 7 to 10 tubes of rounds. I believe a heavy paper tube was also used. Monterey Jack Brass and I have discussed loading three at the loading table and then loading a tube full'o'seven on the clock. I have not done this yet, but it would sure speed things up.
Good luck Amigo,
Panhead Pete