Any ideas?

[Shiloh]

Found these Along the Alabama river in south Al. the flat brass piece has the #’s 60-40 on it.

 

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[creskol]

It looks like an old creosote pole date/classification tag.

 

[Rookster]

I agree with creskol.

 

[firemanphob]

60/40 is a common solder mix. Could have been on a spool of plumbers solder.

 

[Anduril]

I'm going to say "metric ruler".
60-40 = 20, and 20 mm is just under an inch, as your photo indicates.

20 mm = 0.7874 inches
A better photo aligning the piece to the rule might confirm this.

 

[Backwoodsbob]

What about the other side

 

[Shiloh]

Nothing on the other side.

 

[Backwoodsbob]

No mark at all?

 

[Backwoodsbob]

How thick is it?

 

[Backwoodsbob]

It looks like there are other marks on it.

 

[Shiloh]

No other marks on it. It is thin brass. Thinner than harmonica reed.