What's the legal standard for a "pinning folding stock"?

[TangoCharlie]

If a "detachable" magazine means it does not require a tool to remove, is there a similar standard for a folding stock that has been modified to stay closed?

Just want to make sure I do this right....

[1988]

You can use tools to replace the fixed stock with a foldable one.

The logic is the same.