In the 60s there was a time when a lot of the quality 35mm cameras used screw-mount lenses; meaning that there was a thread on the end of the lens and a thread in the body of the camera, and lenses screwed on and off. The nice part about it is that it's simple, and to some extent it became "universal," in that lenses with the same screw thread could be traded between cameras (though there were always exceptions). Lens makers loved it because they could sell one lens that fit a number of camera bodies. Photographers liked the fact that they could buy different makers' bodies and keep their arsenal of lenses intact; they could mix and match.
But screw mounts had problems. It was easy to cross-thread them, and then the lens would be jammed in the camera until a repair tech could extract it and clean up the threads. The threads didn't stop the lens at exactly the same spot, making electrical connections tough to match up. And the natural twisting of the lens to focus it make it very easy to loosen the lens from the mount. So at the 60s came to a close, a lot of the major manufacturers changed to the bayonet mount.
These things seem to seek me out like stray cats. I picked up one at a camera swap and another was a gift from a friend who loved it, but it was obsolete and she wanted to retire it to a good home. (I also had the chance to buy another from a friend who loved his Mamiya, but wanted more than I was willing to pay). At last count I have three 500 DTLs, one working 1000 DTL and another in the parts box. There may be more than I'm not aware of.
Here's a thumbnail comparison of the cameras. The 500 TL is the base camera and it came out in 1966. It's all mechanical with a battery operated meter, so it's heavy but it's built like a tank and reliable (and it can run without batteries). It took (with some exceptions) the Pentax/Praktica universal screw-mount lenses, which meant you could put a lot of very good (and some crappy) glass on it.
The 1000 TL was a 500 TL with a self timer on it (you can see the lever mounted in camera's right front side).
In 1968 Mamiya updated them both with a dual-metering pattern feature, and called them the 500 DTL and 1000 DTL respectively. The new feature allowed the photographer to switch from averaging to spot metering via a little button on the camera's lower left side (on the lens mount).
One of Modern Photography magazine's Top Cameras
Camera manual: Orphan Cameras.com