I've had many mechanical watches where some were/are COSC and others not. All performed within COSC specs regardless of certification with the exception of some vintage pieces.
By far however, my Tudor BB is the most accurate I've had at 0.2spd.

 

There is an existing thread about this.

 

This one, runs a second a month.

 

The title of the thread includes the word MECHANICAL! Not quartz! Stick to the topic!

 

Just shows how much you don't know.

 

Lol.... Thought of typing the key word in caps.

 

Oh snap. Thanks.