Alternator Wiring-2nd Question

[Shoebox]

While reading up on the installation of an alternator in my '51 Tudor converted to 12v neg. ground, I came across the mention of a situation where a car may not shut off with the original ign. switch due to current back flowing because alternators produce current even at idle,...... OR SOMETHING LIKE THAT Anyhow, a component called a DA Plug was recommended and I got one. 18 years ago I did the same 6 cyl to V8 swap that I am doing now on a '51, and used a 6v pos. ground alternator, and I don't REMEMBER needing such a component. I must confess, there are many things I don't remember from 18 years ago (or what I had for lunch yesterday) but I am wondering if any of this makes sense to those who have done something similar? See pic of this DA plug on my alternator. Thanks guys!