Sterling silver belt buckle, or is it?

Marked 1-3 Sterling. I have another sterling buckle by a different manufacturer with the same patent. The other is marked just sterling. I’ve never seen 1-3 sterling mark and don’t know what the 1-3 is. May not even be related to the silver? Either way this one looks like it’s silver plated and worn to brass. There is some serious verdigris which would point to this. I’ve seen verdigris on silver but not like this on sterling fineness. Thought for a second it could be worn gold plating showing the silver underneath but wear location isn’t right. Has anyone seen something like this? I don’t think Knothe was making counterfeit sterling so guessing 1-3 relates to plating?

 

looks good from here...
but the part marked gs...is steel...or other metal

 

What does the front look like?

 

I've had a couple of these.....Hicock...if I remember..
the front were engine turned with initials..or cur out florals.....
like these...







some are plated...some silver front...some sterling..

 

every edge or heavy wear spot appears to show exposed brass. The spot marked gs shows some wear to brass as every other edge in the back plus a huge splotch of green. The front looks pretty good for silver. Maybe 1-3 mean the top 1/3 is silver lol.

 

Front looks good. Edit: well that S looks a bit wonky but it looks good for silver.