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

I have mentioned before that the original rifle has some cast to the stock.  I decided to duplicate this cast on this rifle.  The original had 1/4 inch at the heel, or rear of comb, and to the right.  The toe, or bottom, had 1/2 inch, also to the right.   

This 1/2 inch is a lot, I actually reduced it to 7/16, but even that should make for some interesting shaping.

Photo 1. The buttplate is set over the heel cast line.  The other is the center line coming off the action center.

Photo 2.  The buttplate is setting over the toe cast line.  Again, the other line is the center.

If you install a buttplate on the centerline of a stock, the buttplate is fitted square to the center line.   

The same holds true for a stock with cast, except the buttplate is square to the new angled cast line, not the centerline.  With cast, if you were to install it square to the centerline, but on the cast line, the plate would not be on straight to the finished stock.

Photo 3 shows the line that is square to the cast line.  This doesn't look like much, but it would be seen if not done properly, especially with a long tang.

The problem with this stock, is the deference between the top and bottom cast being so different.  In this case, my plan is to follow the greater of the two angles, or the bottom toe line.  The top  tang, being shorter, should cover up the extra tipping it will have.  If I lined up on the top tang, I'm afraid the longer lower tang would look crooked.
Bob 

So this locating pin hole is also the final screw hole position, the pin just ends up being the same as the screw without a head.  I also only used the top screw position for the locating pin.  It can be installed straight, where the lower screw is to be installed at an angle.  That second screw would not allow for easy removal during the fitting.

I do this in the mill.  This first picture shows the setup...nothing complicated.   

I want the buttplate to go, on and off, straight off the wood.  This is the reason I used a level across the two humps.

This next picture shows the buttplate in place and clamped into position.

Photo 3. Like I said before this hole is also the final screw hole, so the hole is completely finished for the screw, but used with a threaded pin for fitting the plate.

Photo 4.  The locating pin in use.  Getting close to the final fit as can be seen by the black on the wood.

Photo 5.  The final fit and now ready for the screws.
Bob

The top profile is now roughed out as seen in this first picture.  I have also drawn the cheek-rest in, getting ready for grinding.

I use a 4 inch angle grinder, with a sanding disc, to remove the excess wood and get the modeling started.  It's easier to get the wood off this way rather than with a rasp.  I also like to have the wood at the action frame reduced before grinding, then I don't need to get close to the metal with the grinder.  This picture shows the reduced sides at the front.  Can't see them in this picture, but there are scribed lines on each side that are .100 wider on each side than the frame width.

I thought this third picture was a good one showing what the top of the stock looks like at this point in the shaping.  You can see there is a small amount of wood to remove to get to the final size.  This wood will be removed, with rasps, during the final modeling of the stock.

This last picture shows more layout lines.  This Rosewood is so dark I need to scribe the lines in order to be able to see them.  These lines are wide enough to leave around an 1/8 inch extra  on each side, after grinding.   The extra wood allows for final shaping.
Bob

Thanks Joe.

Got the grinding done today.

Pictures of both sides, top and bottom.

The last picture is the bottom, and I took it like this in order to show the cast better.  I tried to line up the action sides with the picture so that the cast really stands out.

She's going to look pretty strange sitting in a rack with other straight rifles!
Bob

Is it just an illusion, or do you plan for a fair amount of cast off in this stock?