A quick google search will show you images of those seals. Basically it's an easily breakable piece of thread that is looped around the end of the last ripcord pin just past the closing loop (or cone) then wrapped around the base of that pin, it ends up being a loop. The thread passes through the small lead thing that is crimped onto the thread similar to a small fishing weight crimped onto a fishing line. When the ripcord is pulled, the thread breaks. The lead seal might fall off completely, but it might stay attached to the ripcord by the piece of thread wrapped around the base of the pin. If Cooper no-pull bounced, the seal is probably still there. If he pulled, he most likely tossed the ripcord, and it would be nowhere near wherever the rig ended up. If someone found the ripcord, the seal might still be on it. Reserves (and pilot emergency rigs) have to be inspected and repacked at regular time intervals, and usually they have not been used. The seal would still be there, but it would break when the container was opened. The rigger would throw away the old seal and replace it with a new one when he was done with the repack. If the chute was actually used in-air, whether the seal was ever seen again would mostly depend on whether the jumper kept the ripcord. Student skydivers are taught to toss them, experienced jumpers usually try to keep them. A pilot doing an emergency bailout would likely toss it.
I thought the serial numbers were known. Both of the packing cards were left on the plane. Flyjack posted the numbers many times.