Geez Ken, that was fast. Finding the right dimensions is a constant battle with a lot of the drawings in the Carreras collection. I suspect that some of them are left for the drafter student to figure out on their own. I recall seeing that in Tech College a life time ago when I tool CAD Design Drafting. The instructor would point out that some of the dimensions on various problems were either missing or incorrect and we had to use our best judgement to figure them out.
The one issue I have on this part model is trying to interpret where the tabs intersect with the rim of the body. In both views it appears there is some distance between where the outer side of the tab intersects the top of the rim and the ID of the rim. Using the given dimensions I don't see how that is possible.
The counter bore dimension line you mentioned doesn't even align with the c-bore bottom so your solution is likely correct, And the screw that in installed in that hole is item 51 on page 13.
Here is a link the PDF book that the drawings are taken from, I should have attached this before, it contains the remaining parts for the model as well as other model drawings.
And here is a link to the web site that has more of the work of T Carreras Soto:
www.fastgrup.ro