Just figured it out. The “collar”was the roller bearing race! Duh. Got it removed and new bearings and races installed. Got the pinion bearings on and planetary gears in. Thought shims were needed until I checked the parts box and found adjustable “shim caps??”. Very cool design. Can screw in and out to adjust. What are they really called? Now the tricky part- setting pinion depth and ring-pinion adjustment. Any suggestions?