Slick Mag Question

I'm working with a friend on his RV-10 project. I was looking over
the engine parts prior to assembly and I noticed something on the left
mag that I wasn't familiar with.

On the side of the LH mag, there's a small stud with a couple of
nuts and washers on it. The label reads : Retard. 13-15 in./lbs.
Nothing similar on the RH mag. My guess is that is has something to
do with the starting system (clue: LH mag only). These are "shower of
sparks" mags and have no impulse coupler.

What goes on that "Retard" connection? Is that some sort of power
lead connected with the shower of sparks? Almost all my experience is
with Bendix mags with impulse couplers.

John Galban=====N4BQ (PA28-180)