Western cleaver construction question

[kjc-01]

I have a carbon steel Briddell cleaver that has a clear line along the blade roughly parallel to the spine. It is even more visible after a mild forced citric acid patina. Is this a border between two different steels or maybe a quench line to give a harder heat treat on the cutting side and softer along the spine?

 

[Auglaize117]

My guess is a quench line. I say this because I doubt they were making cleavers in San mai. I could be wrong, but that's what makes sense to me.

 

[tltt]

Quench line. Solid steel (mono-steel) was a selling point for cleavers, etc.... when one like this was made.

 

[Dr Snake]

Interesting, I've never seen a quench line on a Briddel cleaver. I thought it looks like staining from soaking a tarnished blade in vinegar. I did that once to knife blade and it gave me that same two tone effect.