The "flexible guard" is one of the oldest known types of pince-nez and they are often found on eBay and other online sites. The "flexible guard" style predates the hoop spring and it was popular from about 1870 through the 1890's. This isn't a comfortable style to wear but it is historically important and an interesting collector's item.
The "flexible guard" type pince-nez were among the first type of eyeglasses. It is important to remember there is a distinction between spectacles, which had temples or arms, and eyeglasses which had none. The distinction in terminology lasted until about 1930 when pince-nez replaced eyeglasses and and the term spectacles begin to disappear from common usage in the USA.
The earliest successful eyeglasses (pince-nez) for full-time wear were a precursor to the flexible guard type dating back to the Civil War era. These glasses, shown above, were made of a hard rubber type frame with a very high arched metal or steel spring bridge. They had no real nose guards but slight tiny protrusions as part of the rim. These protrusions pressed against the flesh when the wearer clipped them on just below, and to the widest part, of the nose bridge. This was a one size fits all model pince-nez and they almost always had a handle or slot for attaching a safety device (i.e., cord or ribbon).
This precursor shown above evolved into the flexible guard style pince-nez about 1870. The nose guards were similar to the hard-rubber type in design but this style was of metal construction. This offered more flexibility, thereby offering more comfort and security. Portrait photos of the period show this type of pince-nez with or without safety devices. A good example of the flexible guard variety is pictured below.
The offset guard (aka hoop spring) type pince-nez was tremendously popular from the 1880's through 1900 and remained in public wear even a bit longer. As late as 1908 it was available in the Sears Roebuck Catalog. The pertinent page is shown below. Please click on the photo to read the text as it is very interesting and you can compare the hoop spring to the flexible guard as both are featured on this page. Sears recommended the hoop spring for full-time wear and the flexible guard for occasional use. The Renaissance concurs.
The introduction in the late 1880's of the hoop spring pince-nez saw a decline in the USA of the popularity of the earlier pince-nez. However, the flexible guard remained in widespread use in Europe until decades later.