That layout looks very similar to Lada 1200s (Fiat124) that I used to work on in the 1970s apart from the stick being shorter, there is a plastic cover 8mm screwed to the transmission tunnel, that when removed allows more access to the top of the gearbox.
Just a thought could there be a broken spring below the gear stick in top of box that isn't holding lever away from reverse selection possibly. See if gear lever still rotates if held up a little.
I seem to recall removing that cover and the rocker cover and ex front pipe to drop gearbox down to do clutch jobs in half the book time. No bonus sadly.