When did Longines start to use signed crowns?

Today my "new" Longines arrived. It´s a very early 10.85N ladies trench watch that I will show in a extra thread.
Serial is at 2.8 mio range so back from 1912-13.

On my first inspection I had a closer look at the hardly worn crown.
Quite surprised when I saw a signature on the crown that looks like a large L.
I have never seen a signature on these early crowns...

So any ideas about that signature and when Longines has started to use signed crowns??

This "L" does not look original. Signed crowns began in the 1950s.

Thought the same, but for what reason someone adds an engraving on a watch crown?? Big boredom, or did a watchmaker wanted to be sure to reinstall the matching crown?? Strange & funny...

L for Livingston? Luisiana? Lilliput? 🤦

Lots of marred crowns around. Below is one I used to own.

I was actually wondering the same thing with a 1951 Longines that I purchased recently that did not have a signed crown. From the little research that I did, DirtyDozen12 seems to be correct that it was sometime in the 1950s.