I just R&R'd the advance unit. Here's what I found upon DISassembly:
I removed the center nut/washer and slid the cam shaft out through the points side (it looked fine):
I removed the cir-clips holding the weights/springs and slid them off their posts. As you can see, the raised flange on the notched nut is obviously facing inward, so it cannot be seen from here. Inside the hole, you can just make out the end of the cam shaft and its key (around 7:30):
I don't think you guys requested that I remove the advance backing plate, but I was going to just to see what's behind it. Unfortunately, I couldn't get this screw-thingy off (it wouldn't pull or twist off by-hand, so I left it on). You can see the little positioning pin(?) to the left of the screw-thingy, which I reckoned is supposed to sit between a pair of notches on the notched nut to lock it in place:
As you can see, there's a scrape mark/indentation where that positioning pin ground against the inside face of the notched nut:
With the advance components removed, other than the pin in the cam shaft (which looked fine), I didn't see any other pins or keys around the hole in the advance side where the cam shaft fit through. So I began reassembling, tightening the notched nut until that positioning pin was locked as you see it here (note: the flange is now facing outward, towards the camera, per your suggestion):
Then, I installed the round spacer with the engraved arrow (note the key in the cam shaft is properly located in the round spacer's keyhole):
I lightly greased the weight posts and installed the weights/springs, followed by the weight cir-clips (not pictured). Note: the engraved lines in the backing plate and round spacer are aligned as they were originally:
After that, I retested the timing with the timing light and found it off. So I re-gapped the points to .35mm and re-tested with the timing light. Still off... So I tried re-gapping the points with the dwell meter and was able to get both sets pretty close to the 11 degree line on the dwell meter (set to 8-cylinder - remember, my current dwell meter offers only 6- or 8-cylinder settings).
Then, I re-timed it with the timing light, which went well. Trouble occurred when I increased the engine RPMs above idle. By the time I got to ~2300rpm, the engine (I think mainly the left cylinder, but not positive?) began back-firing. I did a quick timing light check and noted that the timing marks were just sitting between the "F" lines and NOT advancing as they had done prior to today. I pushed on the weights to be sure they were moving freely (they were), so I think either that notched nut was installed correctly originally or I must've done something wrong when I reinstalled the advance unit setup. Though, as you can see in the photos, there's not much to get wrong???
Is everyone sure that notched nut is supposed to go on with the raised flange pointing outwards? That's the only thing that's different since I timed it yesterday (and the timing marks moved as the RPMs increased).