My '69 had the chrome lid as part of the Firebird 400 package. At some point yours might have been replaced with an aftermarket air cleaner, as mine was, and simply buying a stray factory air cleaner might have been an attempt at "restoration".

Here's a pic of a "PR" with the chrome lid (factory photo for dealer brochure).


As another note, the dual vacuum advance was functionally disabled by a TSB. It is possible that yours may have been replaced prior to the TSB in an attempt to fix the issue the TSB addressed. Vacuum diagrams are included in the documents below.
http://firstgenfirebird.org/dtb/docs/68-T-2.pdf
http://firstgenfirebird.org/dtb/docs/68-T-2A.pdf
http://firstgenfirebird.org/dtb/docs/68-I-85.pdf