Tim,
Thanks for the response, and sorry for the lack of clarification concerning the "red book". Yeah, I was referring to the Guild's J & D workbook.
What you describe in your second paragraph matches up with what Sobon goes on to describe, that the commons were tenoned into this ridge and bear on the plate.
This led to a few more questions.
1) Why weren't these ridge beams posted?
2) If these non-posted ridges did nothing to alleviate thrust at the plate, why did the old builders use them, and why not simply m&t the commons to each other?
I'm sure there were some structural benefits that I'm just not seeing.