The German firm Telefunken had been doing parallel development and first offered a mouse, called the Rollkugel, in 1968, although the earliest verified installation we know of was in '72. They did not patent it for whatever reason which may have interfered with its recognition as an early mouse.

Perhaps the earliest implementation of the notion of clicking on stuff was the enormous, networked Air Defense computer system from the 50s known as SAGE, the Semi-Automatic Ground Environment. It did not use a mouse per say, but a light gun with which the users "shot" interface elements. Some say that the introduction of the ballistic missile made it obsolete by the time it was installed, but it remains the progenitor of not just the GUI but also networked systems of both LAN: each site had two complete systems that operated in parallel to provide fault tolerance; and WAN: all systems across the country were networked together and could operate remotely with all functions available through the network, in at least once instance a site burned down and the second computer at the site my father was stationed at was able to take over for them, they ran that way for over a month while the damage was put right. The systems were individually fault-tolerant as well, technicians could even change faulty vacuum tubes while the system ran. It had a hard drive of sorts, one of the early spinning-drum type drives that were typical of very earl such systems, although I believe it used it in a slightly different fashion to modern hard drives. The main memory was ordinary core. Other potential firsts for SAGE include its capacity to play music on its many alarms and a program to draw a naked lady on the vector scope -when you "shot" her navel with the light gun she danced, so perhaps first computer porn as well.

My father served in the Air Force as a weapons controller, a role in which he used SAGE and this light gun. He came to be quite good at it, and eventually was tasked to teach others how to use the system. When the time came to develop a replacement, he sat on the panel designing it. He liked the light guns and lobbied for the next system (I want to say it was the BUIC but I seem to recall the last time I talked about it with him before he passed he said it was another system I don't remember) to use them, but trackballs were chosen instead. I don't know the specific reasons for the choice but I do know that the reason light guns and light pens fell out of favor generally is that frequent use of such devices was liable to cause a condition known as "Gorilla Arm", a type of cumulative trauma injury from reaching up towards a display monitor to use the pen or gun.

When SAGE was decommissioned, the Air Force got back more money in scrap, especially gold, for the system, and that was with practically everybody who could taking whatever was not nailed down or red hot, my father saved some panel buttons, the illuminated panel that marked his station, and an ash tray from the console. He was not able to get a light gun, and I tried to find one right up until he passed in '22 but the best I could come up with was someone selling the pickup tubes. One site out West got decommissioned early, and a great deal of its parts found their way into Hollywood movies as props, for instance you can see some radar scopes in Airplane! in the scene where a guy is putting his laundry into a washing machine set between the scopes. Invasion of the Bee Girls is the only other film I can think of at this time but there were tons and SAGE parts are probably still in many studio and independent property houses.