They are both 81 drive link loops of RM3 chain, green label 0.325 pitch 0.063 gauge. The #3634 is on the Stihl website and in my paper catalog 2012. My guess is that the 3689 is an earlier variant. Maybe it is like you say in post 4 one is distributed in clear plastic and the other is made from loops at dealerships and put in cardboard boxes. From the pictures (ones I found in about 5 minutes) the bumper drive link and the features to help oil seem the same. I got a roll on non current number 1/4 inch pitch chain 13RM 3856 not the 3660 in the book and have not worried about it.
If it was a roll the last four digits in those numbers would be the number of drive links, 1848 for 100'