What is this upside-down silver key icon
It's signifying a unique constraint. In SQL Server, a unique constraint is actually implemented as an index under the covers; this is why it appears under the Indexes node as well (I can't give you a good explanation of why it is not at least listed under the Constraints node, nor why they chose an upside down key instead of just a different color). 

You should see a result here to validate that this is actually implemented as an index:

SELECT index_id, type_desc, is_unique_constraint
  FROM sys.indexes
  WHERE name = N'IX_tblCustIndex';


You said that you saw "Script Constraint As..." - if you followed through with that it would have provided you a script something like:

ALTER TABLE [dbo].[tblCustIndex] ADD UNIQUE NONCLUSTERED 
(
  [some_column] ASC
) WITH (...a bunch of default options...)


But don't run it because this syntax does not use a name and it will just create a second, redundant unique constraint...

Here is a very simple repro, and proof that I get the upside-down key even with very basic syntax, on 2014:

CREATE TABLE dbo.foo(id INT UNIQUE);



   click to enlarge


Now, you can get a different icon if you create a unique index explicitly (and the only differences you'll see in sys.indexes are the name and the value of is_unique_constraint):


   click to enlarge


For other differences see this Stack Overflow answer.