You're not alone... Same happened to me when I bought a V Elmar 100 (enlarger lens) ... even smaller and more hidden than on Summaron...
Just a bit of advice: If you are looking for a reasonable loupe, that is not that costly & is high in quality:
Go to a Jewelers/Jewelry Supply Shop.
Ask to see some 18mm, 10X, Hastings Triplets. Make sure that they have GLASS lenses.
Ask the person to let you look thru a number of them.
Pick the 1 that has the sharpest image at the EDGES.
Since they will all be fine in the center.
Because, like with photographic, or other types of optics: The higher the quality of the lens design or individual sample:
The closer the image quality nearer the edge will match the image quality in the center.
The reason that I gave the above specifications for a loupe is that this is the standard loupe of the Diamond Industry Worldwide. You can find larger diameters, as well as well known brands such as Nikon, that may be somewhat better. Altho incremental improvements in image quality, etc sometimes means a leap exponentially in price.
Because, since it is the Standard: A lot of manufacturers make a lot of 18mm, 10X, Hastings Triplets, with glass elements, in a competitive market.
Generally the loupes that I wrote about above will come branded with the name of a Jewelry Supplier.
The loupe will be fine & rugged uncased in your pocket.
If the loupe comes with a FREE case: Take it & use the case for something else. The loupe does not need a case. Also: The cases are not as rugged as the loupes. They don't last.
If someone asks you to buy 1: It is better to politely decline.
My current loupe has been in my pocket being bounced around with all sorts of things since the 1980's.