It's the strength of the pin. This is from a reliable source from GEC.
"Brass bends/yields @ 88% load than nickle silver."
A couple things to note here:
That might be why so-and-so reputable-whoever said they did it’s misleading in two ways.
1) the pin fails in direct shear (not in bending), which for 360 brass is 37,700 psi.
2) a brass pin with 1/16” diameter, going really small for conservatism, can take 116 lbf in shear. The load is shared by two sides, so that’s 232 lbf the joint can take.
I happen to have a #92 in my pocket right now:
Using the #92 master spear blade for the geometry, I figure the force at the tip of the blade is approximately 8x at the joint.
with a piddling 1/16” diameter 360 brass pin, you can apply about 30 lbf directly to the tip (perpendicular to rotation) before it would fail.
1/16th brass pin -> 30 lbf
1/8th brass pin -> 116 lbf
I’m not sure what they’re worried about, but I’m confident brass can handle whatever nickel silver can in a slip joint.
maybe one of our knife makers could chime in here and provide some more information, but I think that’s pretty good for back-of-the-envelope.