Thats when the first three cylinders of the firing order fire thru one pipe and the last three cylinders fire thru the other pipe, or in the case of a v-6 or opposed six when one whole bank fires thru one pipe and the other bank fires thru the other pipe. Once you hear it you'll recognize it.
All of your post is spot on IMO with the exception of this part which I understand as being a bit different. The first unusual thing on the stovebolt head is that there are 6 cylinders but 4 ports exiting the head. 1 and 6 are separate but 2 and 3 share a common port as do 4 and 5.
With a firing order of 153624 and a split manifold or headers the following happens:
1-single port, front pipe
5-shared port, rear pipe
3-shared port, front pipe
6-single port, rear pipe
2-shared port, front pipe
4-shared port, rear pipe
Pulses alternate front/rear (aka right/left) every spark but also the port type changes every second pulse generating that unmistakable sound.
