The 516 was a flat pack type. the phones looked like EK but were different. EK 820/1232/2264 was a square box with slide in cards. it was the cheaper side. The Ultracom 820/1232/2264 was the bigger dog. phones looked different and was a 3 wire system with off hook
announce. I could not carry a 2264 power supply up 1 flight of stairs today. Most likely could not even pick it up. But I put in a load of them in the day.