Agreed Larry, this is a late gun.
It has almost never been fired as witnessed by the breech face.
These guns are very , well, robust... and will last a life time.
$650 is a good price in that shape. I would use it any time, especially in bad weather.
The "Robust" was the utilitarian grade and was used by farmers on a quasi daily basis.
The treatment is indeed cyanide, and not charcoal case color.
Best regards,
WC-