Yours is the third edition and the first published by Shopsmith Inc. sometime after they formed to make the Mark 5. There is no copyright or publishing date for this edition, only that it is from Shopsmith Inc.
Now I stand corrected.

As br549 pointed out the 3rd edition has the measurements for the joint correct and upon further inspection the 2nd edition is also correct. The revised and 4th editions have the error. The original PTWFE has a 1/2" measurement whereas the rest of the editions have a 5/8" measurement and depending on where that 5/8" measurement goes, either to the bottom of the first cut (correct) or the top of the first cut (error), makes the difference. When I first looked I only saw the 5/8" measurement being the same in editions 2, 3, revised and 4 as compared to the 1/2" measurement from the 1st edition. Clear as mud now right?