smnbin wrote:
I'm using an Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter, but I guess I need an extra adapter?
Your top photo shows a video connector of some sort (top), a USB-A plug (middle), and a MagSafe connector of some sort (MagSafe 1 or 2) (bottom).
To the best of my knowledge, there aren't any adapters from MagSafe 1 or 2 to MagSafe 3, so you would not use the bottom connector. You'd just wrap it in a plastic bag or something to keep the pins from touching any metal object that might short them.
The USB connection is presumably to connect downstream ports on the monitor, and/or to let the Mac talk with the monitor to control things like brightness. A Thunderbolt Display (A1407) would presumably make its data connections through the same Thunderbolt cable that carried the video, so this seems to be visual evidence that you do indeed have a 27-inch LED Cinema Display.
If the top plug is a Mini DisplayPort plug, it should physically fit into the Thunderbolt 2 socket on the Apple TB3-to-2 adapter, but since the adapter only translates Thunderbolt protocol, there won't be any video signal.
I'm guessing that you want something like one of these:
https://www.amazon.com/s?k=usb-c+to+mini+displayport+adapter
Usually, I'd suggest a one-piece adapter cable, but if your Mini DisplayPort cable is permanently attached to your monitor, then you'll want one of the adapters that goes from USB-C to a Mini DisplayPort socket.