Here are some nice pix from the 1950's... if I had to hazard a guess, I'd say 1956, or possibly 1957.
First up is this really wonderful shot of the Rivers of America. It almost feels like it's from an MGM movie, there's so much activity. You've got Huck Finn's raft making the crossing, heading back to the landing near the Plantation House, by the looks of it. There's the short-lived riverfront bandstand to our right, and the Mark Twain, looking as grand and ornate as ever. A Conestoga Wagon can be seen on the distant shore, too. Love those barren hills, maybe they've been scrubbed by hydraulic mining techniques. There's no Cascade Peak yet.
This one was labled, "Thelma and Old Boat". Old Boat? Outrageous!
Now that I think about it, I would expect to be able to see Skyway cables (or even a few gondolas) overhead, but I guess it's possible that they are just out of frame. One of Casey Jr.'s smokestacks is just visible to our left, and some of you will be happy to see the telephone pole and wires!
I'd like to give all my readers a heads-up for a great new post over at the wonderful Long-forgotten blog. HBG2 shares some recent discoveries regarding Ken Anderson's early story concepts for the Haunted Mansion, including tons of images. It's really fantastic! Do yourself a favor and go check it out, you'll be glad you did.