I don't know California law, but there MUST be some sort of 'Collector's Law' in California.

In Canada you can own any kind of firearms as a collector so long as they are rendered inoperable, such as pouring cement down the barrel or removing parts such as the hammer or the firing pin. I suppose technically this means you could legally own a rifle and then just stick the firing pin into it when you needed it, etc. But I won't get into that.

I would assume that you could legally own a WW I knuckleduster trench knife if you had it mounted in a display case. After all, this renders the knife impractical for use as a weapon.

The law revolves around the 'knuckleduster' aspect of the knife as a form of prohibited weapon, and it's similar to the laws here in Ontario regarding such weapons. I would think that for any convicition to stand there would have to be an element of 'mens rea' or 'a guilty mind' about it.

If you had a knuckleduster trench knife hidden under your mattress then it might be seen as 'possession of an illegal weapon.' However, if the trench knife was in a locked (or closed) display case in plain sight then it would be clearly a display piece, and not intended for use as a weapon. This would be especially true if there was other WW I memorabilia in or around the display case such as patches, medals, buttons, photos, etc.

You can check with your local Police department in regards to this law.

I have a large collection my self and several are "illegal" but as per. the calif. penal code thats only to carry, sell ( with out a sellers permit/ ) or transport with the intent to carry or sell. a privet collection does not have restrictions. If you wanted do put them on display in public would be different. and does have legal guide lines for that.