Mileage could be right. They have replaced the odometer. Those don't break before 100k, and they have had several trannys installed. You should get at least 100k out of a tranny.
Many of these odometers gernanded well before 100k, let along a million miles. Look at the rest of the car - no way the sterring wheel and seat have 1,000,000 miles.
Many of the early lockup transmissions failed and failed repeatedly. No idea why, but I bought a very clean t t-top Mirada in the 90's with 85k on it and the 6th transmission. They finally put a non-lockup 727 in after the other lockup ones failed but they were so disappoined int he car they didn't keep it.
A lady I worked with decades ago her Chrysler odometer die at 70k, I had one fail at less than 2 k miles. Yes, less than 2000 miles. Going own the highway and lots of clacking noise, then the speedometer kept going and the odometer stopped.