Better late than never,
I have 4 knives in Pearl NYKC, and the Walden Knife Co. No doubt that they are aluminum bolsters. It is hard to show in the pictures but I am going to say I think engraved with the handles on. The reason I say that is if you look closely, you can see the overrun into the pin inside the art. Also at the edge of the bolsters there is overrun as well.
On the other had I guess a contoured Stamp could have been used, but do not know how practical that may have been? BH
Update: OK Fellas, I looked at everyone of my examples. Only the one shown in the pics has enough detail to show. After looking at all of my references, every one illustrates the border of the art closest to the Handle well within the bolster. That said I just cannot imagine in pre 1931 anyone had the capability to engrave each one of these knives on 4 sides. If they did it would have been mighty expensive. So I suppose that on the one I show, it seems very well possible that done by hand that the die missed the mark by a few thousandth's causing what is illustrated in the pics. I do believe the center on the pin was quickly engraved as there is no consistency with the profile.....
One more observation, looking through a loop there are some mighty sharp corners, that would be quite a feat to engrave with a square tool

BH