I agree about the hole-drilling problem with any splayed surface; I had run up against the same troubles back in the 1970's, trying to get the splayed transom and cheeks hand-drilled for a cross-bolt on a naval carriage for the 7-1/2-inch American Napoleon tube.

Drilling lots of holes takes up lots of time. When it comes to drilling parallel pieces of flat material, they can be "marry-drilled" in a stack; this is flat-out impossible with splayed brackets. Parallel brackets could be produced faster for that reason alone, whether by hand or machine.

I expect that the transoms would be drilled prior to trimming the ends to the trapezoidal shape, as it's quicker and easier to set up and drill a squared piece (for my 1:12 siege carriage model, Iwas able to cheat by cutting saw-grooves for the cross-bolts into the undersides of the transoms, as I had no reliable method of drilling the long, true holes required).

On all the model carriages I also had a little difficulty cutting and shaping the trunnion notches on the splayed brackets, to get both notches square to the central axis of the carriage, since they are at an angle, and they have to be worked indiviually, just as drill-holes do.

Regards, John