Brent,
I have seen stampings that look sloppy in the past. That is, often enough, because proof house workers are not craftsman, actually they are perfunctory government employees. Most Darne guns built since WWII are 70mm chamber guns, and more then one of them was marked 65. It happens.
Measure them. Id wager they are 70. If not, send it back, or, deal with it. It is not a big deal to make that gun work, at either chamber length. It received French triple proof when it was new, the exact, same proof anything with a 3 chamber received after 1964. You arent going to hurt it.
The original buttplates that are marked Darne, like that one, are a bizarre organic plastic that involves petroleum and horse hooves, among God knows what other chemistry. They shrink, over time. All of them. There are ureathane replacements available, stateside, that duplicate the look without the color styrations in the originals, that wont shrink. If you are that guy, go that route. I was thinking that gun was mid 1950s, buttplates on Darne guns are a maintenance item when they get that old.
I told you what I would do. Recoil pad is another option.
The metal has the remnants of the varnish that was applied to protect the French grey hardening. It strips right off in a 5 minute bath of lacquer thinner. Ive stripped some, recoated some after stripping, and actually shot my R10 with Brownells baking lacquer. All depends on what you want, and the simplest option is just leaving it alone. It effects very little.
I love a swamped rib. The sight picture is just the bead floating out there in space, no rib to view. That said, not everybody loves them, and I honestly think a raised rib works better for most guys on any clays course. But, I think the swamped rib is the cats meow in cover, after birds. Ive never shot a grouse on a clays course. You get to draw your own conclusions.
The trigger is not reversible. The safety, is. It is a simple matter, usually, and requires no spare parts, just a guy like Geoffrey or JJ, or, me. There can be some complications, but, if you know what you are doing they just slow you down, and add a bit to the bill. The safety, over on the left, is on safe when pointed down, and is flicked forward (it rotates)to take it off safe. There is a spring that keeps some tension on it. You can use the lever as a safety also. More talk about that if you get closer. The safety is a pain in the ass. The guys who quit using Darne guns do so over the safety. The guys who keep using a Darne, adapt, because they want to shoot a Darne. You will figure out which one of those guys you are. Guys who are left handed, (me, Dustin) wonder what all the fuss is about.

You have my sympathy on the skunk thing. My Mother owned an Irish Setter show dog that would hunt, and he found, fought, was sprayed by, and killed every skunk he met in his 10 years on earth, maybe two dozen. Every time he got his head wet, his entire life, I faintly smelled skunk. He found a lot of birds, also.

Best,
Ted