Gearbox.

Are you turning the rear wheel at the same time as trying to select a gear? It makes all the difference to going through the gears with the engine stopped. When you think about it, trying to select all 4 without the box revolving, will result in a hang up as the some of the gear dogs will not be aligned. This is not helped by the fact that the commando when on the centerstand usually has both wheels in contact with the ground. You said the bike is on the bench, is the rear wheel resting on the bench so preventing it being turned? 

My guess is pawl engagement with the quadrant. If you search on here you'll find info on adjusting the spring so it's 'just right'.

Why does it work on the road but not the bench? Probably more that the engine is running and rattling eveything around.

The quadrant is in a different, extreme, position in 4th. You could take off the inspection cover and peer inside with a dentist mirror and then tell me I'm wrong...

My box had a " reluctant " 3rd till i spent some time with a fine stone easing the tight spots on the gear / shaft splines . I had always assumed that after 40k miles these would all be a loose easy fit.,but no.

Reluctant gear changing is often caused by the dodgy springs in the gearbox. Check the two lurking inside the outer cover. The thicker Return spring sometimes cracks. It might look OK from the ouside but the crack allows gear changes in one direction but not the other way.

Likewise and similar to above, the Selector spring has to be a near perfect shape and fitted correctly to function properly. The most common mistake here is to fit it upside down. Check that the straight leg is at the top.  See photo.

The third dodgy spring can be that which operates the Camplate Indent Plunger . Over time, these can go soft and mess up the gear changes. Generally however, this results in just jumping gears.

This photo is typical of what you can be up against. A small selection of AMC  Gearbox springs all purchased at the same auto-jumble. Ony one from each set actually worked correctly.

Agh! Selector pawl springs the AMC 'box Achilles  heal let me down twice many miles away from my comfy bed. 

Sadly many of their excellent 'how to' guides seem to have gone, but I do have a copy of their notes on setting up the pawl spring in the AMC box. Certainly helped me when I was having 'issues' setting mine up last year.

Hopefully the attachment will open ok.