Thought on "It's a Wonderful Life"

I'm a long time lover of the classic Christmas movie "It's a Wonderful Life".

Like many others, I've often wondered about Clarance the angel and the line: "every time a bell rings an angel gets his wings." 

It seems an odd line to come from nowhere.  What was in the scrip-writer's mind when he wrote it?  To have the line actually be related to the overall story suggests that he was familiar with such a belief/custom/saying.

I think I may have found a possible origin.  

I was browsing Wikipedia and ended up in the article on Indulgences   (For those who don't know, the over-simplified explanation is that by buying an indulgence, a sinner could avoid part of the punishment in purgatory for a sin)

Luther, who was a key figure in the Protestant reformation was strongly opposed to the sale of indulgences.  

 In Thesis 28 Luther objected to a saying attributed to Tetzel: "As soon as a coin in the coffer rings, a soul from purgatory springs".^[40] 

I think that quote of Tetzel is the likely origin of the Angel getting his wings saying.