Transmission oil/gearbox Seat Ibiza 1991

Hello! I have a 1991 Seat Ibiza and I am trying to check the transmission oil level. In Systempsche.blogspot.com/ I have found the manuals that it says how to check the level, basically taking out the screw from the gate back and see if it drips, it goes to jet or nothing comes out. Then he tells you that to replace the level you have to fill for the 'filling cap', which I don't know what it is. I suppose it is a different cap from the engine oil.

I have found a kind of rubber cap where it puts 0º, 5th and 10. I suppose it is not this since there is no indication of ever hosting oil.

The reason for wanting to review it is that it lost a few drops of oil by the bellows of gear change, and then the loss disappeared. I want to know how the level is.

According to the manual, from the number of frame 9703472 the oil filling must be made at the filling cap. It is my case since my frame number is 980xxxx.

I attach photos of that rubber cap, and the back cable that goes to the transmission screw and that it is supposed to disarm to check the level.

How do I fill the transmission?