Hi NHGrants,
How lock panels were shaped depended a lot on the makers and style. One thing that is universal though, you can only see one panel at a time so little differences likely will go unnoticed. British makers tended to make the lock and side panels parallel when viewed from the top. Because most barrels they used had a lot of flair at the breech, the thickness of wood along the barrel channel thinned toward the breech to keep the side panel from flaring out with the swamp.
The Brits also were not fussy about keeping the shapes of the panels the same. They fitted the lock panels to the lock and often the side plate panel to the sideplate, as I did below.
Here is a New England fowler treated in the same way
On Germanic guns and many American long rifles one or both panels flair out toward the butt following the contour of the barrel and the panels were roughly similar in shape but I think the old timers were not too fussy about that.
Note on the rifles I built above there is not a lot of flair, although on some original guns there was depending on the shape of the barrel. There is an advantage to the British treatment because it is easier to contour the top of the stock at the breech to merge with the barrel flats than if the stock flairs out. However, widening at the tails of the panels has the advantage of creating a wider more robust wrist.
dave