Hi Bamby boy, and welcome to the Forum.
Coincidentally I am just now familiarizing myself with similar issues.
My T400 Saloon has the electronic shifter mounted on the dash, but features a longer and nearly fully bent over shifter rod that makes the desired travel up and down adjacent legs of an X pattern seem less intuitive than the shorter straight shifter's more obvious implication.
Does your have this also?
I learned from Uwe that you must first shift into nuetral after engaging first gear to become enabled to select reverse, by pushing up on the vertically orientated and linear motion activated separate shift button.
Also, all gears except for first need to be pre selected before clutch throw to allow engagement.
I.e., pushing in and then letting out the clutch pedal triggers the engagement of the gear preselected except for first gear, and possibly reverse.
Still trying to work the reverse preselect or not part out as I only am able to find nuetral at this point at best maybe one of three tries. But that's up from one out of ten from a couple of days earlier.
At this point I can't say whether this apparently diminishing difficulty is due to my increasing skill, the car's mechanisms re breaking in, after a hiatus of several decades, or uncertainty of mechanism, as the bent shifter rod is not shown or talked about in the videos that I have seen, and there is presently plenty of slop (be it intended or not) in the mechanism.
Did you take yours apart because you couldn't find reverse, or did you just decide to check it during the course of a rebuild?
If it be the former, please ensure that you were using the proper technique to select it, before trying to fix it, as it may not be broken, and would be better off left alone.
At present for myself, finding nuetral has been the harder prerequisite (after a mandatory just previous engagement of first gear).
But with only a couple dozen shifts under my belt, with a transmission that has been asleep for decades, I am hopeful of increasing my batting record.
Is your manual in English?
P.S. Where are you, and do you know anyone with any extra sheet metal for my 400?