Since this is the type of topic I'm really interested in, I picked up a parts catalog and operator's manual for the D8L SA which is a 4FB1-up serial # range. The main difference in the powertrain is that instead of the torque convertor, it has an hydraulic multiplate clutch (same type of arrangement as in the main transmission) with two sections - one clutch for direct drive, and one clutch for 1.35:1 underdrive. Each powershift gear is split and the speed progression is 1L-1D-2L-2D-3L-3D per the manual. The hand control is a u-shaped setup like the "regular" powershift control (3F-2R - blocked out 3R?) but instead side to side movement selects direct or under, and forward-back engages the clutch. The operator's manual says while it's possible to make full-power splitter and powershift changes, it's recommended to use the decelerator to reduce the stress on the change and increase component life / improve operator comfort. This general arrangement is like the D3C SA in that the clutch function is provided by an hydraulically actuated clutch. Behind the splitter/clutch control are four external rear hydraulic remotes, the first of which also has a float position. The rear drawbar can swing or be pinned.
One item the operator's manual mentioned is to keep an ear open for track pin squeaks, and when stopped, to check the relative heat on the ends of the track pins. This is apparently to check for dry joints - they'd squeak and/or get hotter than non-dry joints.

Yes, I was surprised when I saw that in the manual because I was expecting to see the switch that you mentioned. The operator's manual and the parts catalog align on the lever so I wonder why, and when, the change occurred. I think you mentioned that the switch had two positions while the lever effectively has three (clutch disengaged in either gear, or engaged in either direct or low) so the actual function would have been different too. Couple of pictures showing the arrangement. According to the operator's manual, moving the speed/direction lever will disengage the clutch. It also mentions six forward and reverse speeds so I guess 3R isn't locked out, although every reference I've seen online says it's a six forward / four reverse machine.