Angel from Montgomery

Regarding the John Prine song entitled Angel from Montgomery, I’ve read online that Angel From Montgomery might be related to a pardon from Governor but was wondering if that’s a standard idiom or just one fan’s interpretation in the fan website:

Angel From Montgomery notes.
An "Angel from Montgomery" refers to a pardon for a prison sentence from the governor. It is also used to refer to a last minute pardon from the death sentence. The phrase originated in Alabama where the capital is Montgomery. 
The way John puts it the woman is living in her own self made prison that she can’t escape from, hence she needs a pardon (the angel from Montgomery). 
— jpshrine.org

Is this reliable ? Was it a real idiom before Prine wrote the song in 1970? There is no mention of this pardon interpretation at wikipedia entry for the song.

PS. Please, before you hit close vote I am not simply asking for a lyrics interpretation, I am asking whether it’s an idiom, and explaining as background where I encountered it! ?