A friend recently posted a photo he took of a sign at the LBJ presidential library that used the word "archives" as a singular noun. According to a Smithsonian Institution Archives blog post, this is perfectly acceptable. It says:
First of all, unbeknownst to most word processors, "an archives" is a grammatically correct phrase. "Archives" is one of those words that can be either singular or plural, though it is relatively common to drop the "s" when referring to the singular. "Archives" can refer to a collection of archival materials, the facility in which the materials are stored, or the organization that collects and maintains them.
Are they correct? And is this a lone exception or are there others like it?