After almost 115 years, the keyboard of this little Standard Folding was in a bad state. Most keys had their label and keyring replaced, probably some time in the 1930s or 40s. The replacement rings are a wrong 'flat' pattern and the labels look similar to those used for the later Corona Special machines. To add to this, the 'U' key-lever had lost its key entirely.
Before untangling the 'tapestry' that is a folding keyboard, a picture to note also the position of the spacers at the rod-ends. Some of the key-levers had rusted onto their rod - or rather the rod rusted firmly into the key-lever aluminum bush. These needed several days of oil and gentle tweaking (aluminum is soft!) to loosen them - giving it more time usually helps :)
All key-levers were cleaned with soapy water. Actually, most still needed a scrub with very fine steelwool to make them presentable. Another rinse after the scrubbing to make sure no steelwool debris remains on the parts.
As the key-labels were going to be replaced anyways, the old replacement-rings were removed. The keys are aluminum disks of ~13.8 mm diameter and 2.5 mm thickness (1/10 inch I guess). In the centre a 2.5 mm square hole. Aluminum is an easy material to work with by hand, it's soft and filed easily - nevertheless a 2.5 mm square hole in a 2.5 mm plate is tricky :-)
Keeping a new keyring handy for checking the fit/diameter, the 'U' was given a new key - the lower key-lever in the image above has the new 'U', an original for reference at the top. Looked credible enough.
Using the many useful images of Standard Folding keyboards online and exploring ~1903 typefaces, new key-labels were drawn in Inkscape. The curvy, 'art nouveau-ish' typeface can also be seen on the patent-prototype from 1903, and was kept for nearly all Standard Foldings. Very period in styling. (See patent-model picture on page 200 of the book "Typewriter - a Celebration of the Ultimate Writing Machine" by paul Robert and Peter Weil.)
Couldn't resist and made it a proper keycard. Here below in fairly high-resolution, in case anyone has a Standard Folding in need of new key-labels. (Many of these machines have damaged key-tops - the originals are made from card with a thin celluloid top-coating. This top-layer wears through over time - perhaps from typing with fingernails touching the keys.)
The labels were made with same method used before on the Erika. The keycard was laser-printed on ivory paper, then sealed with artists-varnish and given grey paint on the back to seal/make opaque. This should make the labels last, makes them resistant to wear and to moisture.
Experimented a bit, and settled on using a 13 mm hole-punch to cut out all the new labels. The same way a set of thin plastic-sheet covers were stamped out. With a set of newly polished key-rings, all parts in place to re-label all the key-levers.
The key-rings were taken from a wrecked 1917 Corona 3. They are not quite the right pattern for a Standard Folding (and not tall enough), but less wrong than the previous and they are from the same company. The rings for the shifting keys are still the 1910 original rings - which is fortunate, because the donor-Corona already had lost a ring and had exactly 28 rings left on it (backspace made 28th).
Some PVA glue was placed on the keys before placing the label to prevent it rotating - and the PVA should not adhere to the aluminum and will make it possible to cleanly undo this restoration later. Firmly pressed down the stack of label and plastic cover with the ring and folded over the tabs with a screwdriver-tip to fix it all in place. Only one tab broke (unfortunately), so a small dab of cyanoacrylate placed there. (Cyanoacrylate is by the way also safe for restoration, it can be cleanly removed by heating it - placed in hot water it will lose all strength and fall apart.)
Then weaving the re-labeled levers and spacebar back onto the three polished rods, the assembly is ready for inserting back into the typewriter. Looking much better (and safe to touch again ;-)