Ok
@Paul Bryant let me post this frpm my phone and then I will go down load from the scope and add it to this.
First the drum with the split roll pin in it and the aluminum shaft that passes the gear signal on.
The off center split roll pin hole in the shaft
I
Shaft and spool together
And the shaft route thru the upper case
ok here is a shot with the scope in thru the hole the shaft goes in. in this view you can see the split roll pin hole with no pin in it. that is because when I pulled the aluminum shaft the roll pin stayed in the shaft verses the spool.
Now I can only speculate your problem is here because if it was I the other end of the spool I would think there would be a problem shifting . the other end also has a split roll pin probably for what I would call the selector sprocket. I would have to look up the parts name. Anyway if you pin is some how broke, the hole in the shaft wallowered out perhaps allowing enough movement of the pin so it would come out of the spool could much rotation up a bit. You might be able to pull and replace the shaft without removing the motor like changing the torsional damper but you will not know the extent or scope of repairs until you get that far into it. I recommend checking the front detent area just in case no matter which way you choose to go.