Thanks for answering, Bob, and the explanation is a clear picture of how you did it. I figured it was done while the whole block was still together, but it just looks like it's a smooth 'swoopy' contour like you might find on a casting or something. 
I like what you make and the process is amazing, but nowadays I wonder how long it will be until we begin to see such things done with the amazing printing processes out there. Adios, tradition. And craftsmen like you that are already pretty scarce. 

Photo 1.  I think this is as far as I will go on this buttplate shaping for now.  Although, looking at this picture, the ends of the tangs appear still too large and I will need to shrink them!

I have the area between the tangs mostly rough shaped.  I like to have this close to being done before I drill any holes for the plate screws.  It's easier to figure screw head thickness and slot depths with this being shaped.   

I also have the circumference of the base shaped.  This gets me to the point where I can install the buttplate and still make any minor shaping adjustments, once the wood is in place.   

Photo 2.  A better picture of the bottom of the base and the recessed area we were talking about in the earlier post.  Like I said, not as pretty as it looked in the other picture!

Also can be seen is the shape of the base circumference.  This is also the shape of the wood stock at the buttplate, so it's important to be correct.  I use a hand drawn pattern to lay this out, but in the end I finish shaping by eye.  I keep hoping my eye sight is not fooling me to badly!

Photo 3.  The buttplate laid out on the drawing, just to be sure it is correctly shaped.  I like a smooth transition from the wood through the buttplate tangs.

Photo 4 and 5.  These show each side and the smooth lines I am looking for before installing.  Any irregularities need to be fix.   

In the last picture, the red arrow pointing to the left side of the base, shows a slightly straight area that I did not see when checking the final shape, on the actual plate.  This may be just a photo illusion, but I will need to check it out anyway.  I also think I see the same issue in the picture of the other side, in the same location, so I may have missed it.   

I do study my pictures, often seeing errors or something I don't like, and end up correcting them.  So, if you are building a rifle take pictures and study them.  You will be surprised at what you sometimes see!
Bob