It is very common to find treadles and hand crank cases that have locks, and are missing their keys. Old keys show up in odd places... garage sales, old sewing machine repairman stored, sewing machine collector gatherings, and on ebay. You need to know what kind of key you need for each treadle or case.
Treadle keys... there are basically four kinds:
1. Standard late model Singer: This is almost not a key.. it's just a flat piece of more or less key shaped metal with a rounded, smooth point, no teeth. This type of keyed case can be readily opened with the blade of a small screwdriver. Shine a light into the keyhole and all you will see is a slot. The key, or the screwdriver, slips into the slot and twists. If you want a permanent key, you can bend the flat end of a nail over as a handle, and grind the pointed end off, then grind the end into two flat sides and shape/grind until it fits into the opening and turns the lock.
2. Square ended key: This one has the head of a key, but
again, no teeth. The end is square shaped. When you shine a light into the
key hole, you can see a square receptacle that the end of the key fits into.
Again, but only sometimes and with greater care, you can can open it with
a small bladed screwdriver if the blade end will fit crosswise, corner to
corner, into the opening. Again, a nail can be bend and the end ground or
filed into a square of the appropriate size and used as a key.
3. Triangle ended key: Same comments as above... shine light
into hole. If you see a triangular receptacle, you need a triangle end shape
on the key. Again, you can bend and grind a nail to work. Screwdrivers are
unlikely to work on this one as they just can't get enough purchase.
4. Real key shape: Some of the older cases, especially European ones, use a real key shaped key that looks like the old skeleton keys, only smaller. Some have only a small rectangle on the end, and for those you can sometimes bend a nail to work, but it's a lot harder. Some actually have rudimentary teeth, and those are real hard, though sometimes a jewelry case or china cabinet keys will work. A few of these types have the shaped key, but with a hole in the end. When you shine a light into the keyhole, you can see whether there is a round pin in the middle of the opening. If there is, you definitely need to find a key of this type, and a key shop may be your best bet.
If you do find keys, save them! You never know when you will find that you have just the one needed for an odd case or cabinet.
Here are some of the keys I have collected:
From the top, these are:
Top Row: first four are all square end keys, usually from older Singer cabinets. The fifth one is a triangle ended key from a White cabinet.
Second Row: First is an old skeleton key that I ground into a square end. Second is some kind of old toy key that happened to have a square end. Third is the common flat metal later model Singer key. Fourth is a flat key with rudimentary teeth... don't know what that's for.
Bottom Row: Two round shaft keys that have hollow ends, for European cases. Wood craft stores sometimes have brass versions of this type of key for use in making jewelry cases... sometimes they work!