So, the 4635 is a manual only reverse locomotive. I am speculating that somewhere along the way, an R/C equipped motor was installed in your loco.
The easy response to your question about the lever that engages/disengages the R/C function: A brass rod with a hook (looped through one of the holes) attaches to that metal lever you see with the 3 holes near the reverse unit. This lever engages/disengages the R/C function. Which hole you use is dictated by the body shell of the locomotive, and where the brass rod goes through the hole in the end of the body. You basically pick the hole that lines up to keep the rod parallel or in a flat plane to prevent binding/friction against the body.
If your cab doesn't have the hole on one of the ends, then you would have to drill one. It's probable that your shell doesn't have the hole because it was sold as a correct 4635. If it does have the hole, then lucky you. Otherwise you will have to drill one, or figure something else out. On the brass rod, the end that sticks out the cab body has threads and a nut that is similar to a textured transformer nut (but smaller threads) is put on the end, so the operator can grab it to pull or push it. Pre 1928 models just had bends put in the rod to grab it, instead of a threaded nut on the end.
Picture below showing a #4685 (R/C version of the 4635) with the rod sticking out the end for your reference.
