The teeth on the reverse idle gear (inside the transmission) are chamfered on one end. Is that from wear or was the gear made that way? If it was made that way, does the chamfered edge go to the front of the transmission box (where the reverse idler meshes with the sliding gear) or to the rear of the transmission box?
The chamfer will face the slider, so that there is no possibility of the gears not meshing when reverse is selected. If the meshing teeth have flat faces and have no chamfer, then the gears, which are stationary (at least they're supposed to be) would need to be aligned perfectly to engage, and that's not likely to occur very often.