Lucky Line 87202 Skeleton Keys - Door Lock …
My friend's 1920s Mortise locks have a "3" stamped on the outside cover. However, both of these keys (the 2-bit and the 3-bit) work. In fact, the 2-bit key (the one without the notch) actually seems to work better. "3" may have been the production line, or something, for all I know.