what does this symbol mean on my Spyderco Endura 4 ZPD-189?

[thegodoflions]

Bought a Spyderco Endura 4 ZPD-189 recently and when I watch reviews on it I don't see the same symbol on the opposite side of the blade where the Spyderco logo is.
I bought it from a very legitimate source so I'm pretty sure this isn't a knock off.
So what does the symbol mean?

 

[Modernflame]

It's Sal Glesser's maker's mark.

 

[thegodoflions]

It's Sal Glesser's maker's mark.

I just realized I should've posted this on the Spyderco forums, Sorry.
Anyway why put marks only on certain blades and not on all of them (of the same model) ?
doesn't that lead to confusion for buyers?

 

[KenHash]

Bought a Spyderco Endura 4 ZPD-189 recently and when I watch reviews on it I don't see the same symbol on the opposite side of the blade where the Spyderco logo is.
I bought it from a very legitimate source so I'm pretty sure this isn't a knock off.
So what does the symbol mean?

Delica 4 and Endura 4 without Sal's marking - Spyderco Forums

 

[yablanowitz]

Short answer: Sal designed most of the early models himself, and he isn't one to blow his own horn. Once his son Eric started designing as well, he started putting his mark on his new designs. In response to many, many questions, he has added his mark to his older designs as well, but there were many hundreds of thousands made before that mark was added.

 

[Sal Glesser]

Hi Godoflions,

Welcome to our forum.

sal

 

[norcalAF]

The SAL mark means you should send the knives to me for extensive testing to determine quality

 

[thegodoflions]

Hi Godoflions,

Welcome to our forum.

sal

that's so cool,

sorry I'm just getting back into knives so forgive my ignorance.
I'm glad to have bought this knife as my first serious folding knife.