Very cool! And most likely it did come from Marshall Siding, since it was taken out of service earlier this year.
Those type of signals are used on sidings with hydraulic spring switches from the main. If the main is clear a yellow (restricting) aspect is displayed. Otherwise the red (stop) is shown.
The other pot signal I have seen the SouthShore use (and nobody else that I know of) is for siding with rigid switches. It is a three-aspect signal with yellow on top and red in the middle. The lower aspect will show an "S" when the main is clear. Otherwise that aspect will be dark.