Tom, the buttplates I make are sort of in between the trade gun style buttplates that can be formed somewhat as installed, and regular cast buttplates. I inlet them as though they are cast buttplates but cannot just saw a profile and start inletting. Instead I get the buttstock to near final shaping wheee there will be full support for all areas of the buttplate. But I watched a video by Clay Smith and he does the same for his Carolina trade guns also known as type G. Unlike the simplest trade gun buttplates he rounds the “foot” or area between heel and toe.
Regarding the ballpeen hammer modifications; most ballpeen hammers are flat faced on the one end and a sort of rounded conical on the other. I round both faces and only use the hammer for “smithing”. No nailing or whatever.
I’m showing my try piece here, made of thinner 0.050” thick stock. This is at the lower limit for cupped heel buttplates. I have a lot of that stock I got cheap so use it for try pieces, the odd trade gun buttplate, thicker thimbles, and engraving practice. You can see where I start beating the buttplate into the female form. Yesterday I had to glue up and dowel the form because it split from the violence. In STL I had a lot of white oak available and that made the most indestructible forms.
Hope this helps and ask more questions if not clear. Also showing another buttplate made this way.