Can Someone Help With A #510 Please

I have #510 but don't know which it is because the color chart does not have a brown yellow sample for the #510a stamp.

I'm assuming you're referencing the first stamp? #510 is highly susceptible to sulfurization and can cause one to look like #510a because it has darkened by the chemical reaction. Quite some time back I had one looked at by three expertizers: the first said, "maybe", the second said "Nope, its isn't 510a" and the third agreed and gave it a peroxide bath for me to clean it up. You live and learn.... Yours look like a "maybe" to me.