Tony, that is the best explanation I have heard for the use, or replacement of an outer brass tube.
This goes back to the cleaning of muzzle loading firearms, washing and flushing out arms from the muzzle end to clean them.
I always removed the mag tube, dropped out the breech block and cleaned the Spencer's I have owned from the muzzle end, with a dry patch through the mag tube when everything else was done and be reassembly. (but that is just me!)
The Troopers of the time would not have been taught this.
My best,
Blair