Like Cari said, these come in several different sizes, and they can be hard to tell apart.
There are two basic widths of slot, the narrow fits Free/New Royal, the rest all use a wider slot.
Some have the needle hole offset from center on the slot (can be left, right) White and National are both centered, but White has a 1/8" longer distance between slot and hole. Older New Home are like National, but normally have a really short foot, while Standard has a tall foot.
The ones Lynda posted are often sold as "universal" as they can fit several different machines because they have an open slot for the needle, and not just a whole. However they wont' fit every machine. Her's probably fit a National made machine (and don't you love the part that is used for mounting a light fixture to a ceiling box- I see those so often, along with drapery related hardware with sewing machine attachments).
Lydia also has a mock hemstitcher attachment that is bed mount (and she's got the square mount that is often missing) along with the tucker that has been disassembled and is in four peices, but I don't see the screws need to put it back together.
Some pictures showing some of the variations. (ignore the different sizes of the hemmer part)
The needle hole is lined up on these, notice the differences between the needle hole and the mounting slot. Some have a slot cut in them like all of these, and that is normally lined up with the needle hole. Only one of these four is off center. Most often you will find them without that cut.
This hows the open toe style, notice one has a narrow slot (I don't have a narrow slot one in the above group, but you'll find them like that too). You can also tell that the needle spot in the narrow slot one is off center from that slot. Some are just a tad offcenter, and some are quite a bit more off center.
