Who makes a 1969 firewall insulation pad that most resembles the assembly line original part?
No one makes one that looks anything like an original for Dearborn or any of the plants for that matter unless someone recently has started making one.
Often owners/builders will take one of the current reproductions that has similar jut layer and then modify it by applying undercoating or similar product to the surface facing the interior in an effort to replicate the original look. Pay attention to the cut edges so that the finishing look appears that the cuts were made after the surface was prepared on the bulk material
Depending on the product and technique its sometimes successful. A few practice attempts will produce better results. For me I found a "rain down" application produced better end results
Are there any images that show detail of the finish applied to the visible surface of an original firewall insulation pad?
Here is an example of a 69 Dearborn firewall pad up close. Uneven no pattern. As mentioned in making a reproduction one of the difficulties is the sharp cut and precut opening edges You will likely see some differences between the pictures and yours since these are from a non-AC car but they do show the texture
(https://www.concoursmustang.com/forum/gallery/16/6-040222183933-16921112.jpeg)
(https://www.concoursmustang.com/forum/gallery/16/6-040222183931-169181497.jpeg)
(https://www.concoursmustang.com/forum/gallery/16/6-040222183931-169182171.jpeg)
One that was pulled out of a May 69 Dearborn car
Try West Coast Cougars for a nice used one.
Nothing currently available from WCCC.