Is seam sealer applied on the bottom of the car (1969), near the back of the floor pans where it meets the rear shock panel?? Reference KMCBOSS's car in this link: KMCBOSS
What factory? Sorry I forgot. Yes it differed It even appears to have varied depending on the model of Mustang
Looking at a April 69 built at Dearborn the answer is no.
This car (I'm referring to) is a 30K mile Boss soe the staggered shock installation is slightly different that other models such as the CJ. In this case a small section (don't ask me why) has a bead of seam seal approx 14" long on one seam but none others
I emailed you a couple of pictures to show the detail
I finally did the obvious and looked at my '70. DOH! It confirms a lot of what I was asking. I didn't look earlier, thinking that a 70 Dearborn car wouldn't be done the same as a '69 NJ car, but I figured it's probably close enough. There is evidence of the seam sealer where I expected. It appears that some of it may have been the weld through type that is described in the manuals, put on as the panels were mated. The area where the floor meets the start of the rear axle panel, there is definitely sealer in there. There is material on the rear seam of the shock absorber support panel, but not the front. Not as much around the leaf spring front mount, but some. Some of it may have flaked off over the years. I don't see any around the seat belt plates, but I may have cleaned that off years ago when I cleaned the undercarriage.
I am the second or third owner of the car, so I know it wasn't re-done underneath (Except what I did).