Got home and took off the cover and it is not as simple as I hoped. The forks were in the right place.
Once I was in there and moving the sliders by hand, the problem did NOT go away. But now that I can see what is going on, I can better report what is happening.

It is stuck in third gear, even when the sliders are in the neutral position,. I was wrong about it being 1:1. Now that I can see the gears turning, I can see that it is slightly less. When I engage third gear, nothing changes. Any other gear and it is locked up. Third gear is not disengaging, even when the 3/4 slider is moved to the neutral position.

It looks like an issue with the synchro. For all the other synchros, it looks like there is about an 1/8" gap between the brass synchro teeth and the gear engagement teeth when the slider is in neutral. For third gear, the brass synchro is slam up against the synchro teeth and I think slightly engaged with them.

At least now I know what to tell the builder when I give him a call