The slides on the SW99 are stainless, the German slides are carbon. What is the slide steel on a 40 cal Walther marked gun without the eagle and N? That is, the few that S&W made for a time without labeling them SW99 instead leaving them as Walther bannered?
Mr. Earl has told us they are stainless, but I don't know. If you do a search for a thread about Why Earl's gun's are priced higher you will find a little more discussion on slide composition.
I would say ... be very careful with that gun as Earl has warned us that it is not structularly sound and may explode while sitting in the box without a round in the chamber. (Just kidding ... you'll understand when you read the thread)