Have they told you what specifically is on that manufacturing plate? By the name, it sounds like the kind of information on the body tag that you say is on the firewall. See https://firstgenfirebird.org/1968-info/1968-technical-information/1968-body-tag-information/

The first page of the 1969 Pontiac Service Manual and/or the first page of the 1969 Body by Fisher manual should identify the plates/tags the factory attached. I don't have '69 manuals to show you but here is the information on the '68's as an example: https://www.firebird400ho.com/mdocuments-library/

If your car was built at a Europe GM plant, the tag may look somewhat different but should have similar information to the US format.
See: https://firstgenfirebird.org/forums.../true/68-coupe-in-france.html#Post320916
And: https://midwestfirebirds.org/?p=4865#more-4865