Ok, several things, check this rectangular bush isn't broken, it probably isn't but can have bits missing, a crack is ok and fairly common,
This is the little cube, you can see a wear step on the top? This is caused by the gear selector linkage running along either side when corroded/rough/unlubricated over time, as the cube is also pushed against the top and bottom of that linkages groove every time you use the 1/2 gate (top) or 5/6 gate (bottom). The cube's hole also gets elongated from doing this over years of use and zero lubrication. Rotate it 90deg. This should be greased every few months ideally to maintain most direct lever feel in the long term.
This is under the pivot for the gear selector linkage, just remove, clean, inspect and grease it also.
While you're here, take a look at the gear selector cables, the gaiters at the ends if original will be perished and typically detached at the clip ends. It's a good idea to try and get some oil down these as this allows weather in, ATF is good, if the gaiters aren't totally rotten you can sometimes use them as a funnel, otherwise get an assistant to work through all gates and gears as you drop some in ;O)
Never heard of this? Part of every service I do, and every clutch, every engine removal, every gear cable etc. Freer = Faster ;O)