The style of checking on these guns is something I have found to be fascinating. I have collected a good many pictures and handled quite a few of the rifles, always looking at the subtle variety of the work done.
While the flat bottomed valley is common, it is not always the style. The same can be said for the flat tops of the diamonds, not always. I propose that many of the rifles show flat topped diamonds only because English walnut is very soft, and the rifles have been handled often in the past 150 years. I have seen in photographs, and in person, that on close examination the flat topped diamonds vary based on how the gun was handled. In places where the owner did not handle the rifle, diamonds are quite sharp. Where the handling was often, such as the thumb of the trigger hand, the diamonds are clearly worn. Even when compared to diamonds just a few rows away.
You can see on Snapper's Rigby where the checking is not worn, the facets are quite sharp.
On this Kerr, the checking is worn everywhere. The rifle was used heavily, though well cared for. The diamonds are soft and rounded all over the rifle.
This Whitworth shows the difference quite well. The same checking on the bottom, where the support hand would grasp the rifle is worn very flat. The top line of diamonds are as sharp as the day the checker cut them.
Here is the checking on a Thomas Turner Military target rifle that is coarse, with sharp diamonds, and no flat bottom to the valley.
Last, this beautiful Edge rifle, where you can clearly make out the difference. The circled checking is sharp pointed, a very narrow flat bottom valley, and where the owners thumb rested the points are nearly worn away. I don't think this is a lapse of workmanship simply based on the high quality of the other work on this rifle.
Overall, I would be hesitant to label all English sporting or target rifles as having one identifiable type of checking. I think the differences in the "Brand", craftsman's tools, and customer preferences, played a large part in how the rifle was checked. The borders can vary as much as the diamonds, though nearly all are bordered. The only exceptions I have seen are the tops of the forearm on military stocked target rifles. Almost all I have pictures of show the checking runs out the top line of the stock. Of course, I could be crazy too.
Oddly, looking through my photos last night I found two different checking styles on Rigby rifles. One was Snapper's, and another I found on an auction site. Both had broken hammers!
I hope this is of help to you Curtis. If your checking is as nice as the rest of your work, the job will be well done.
DAve