Linen testers aren't (striclty-speaking) loupes, though they are folding magnifiers. Traditionally, they were used for measuring the threads-per-inch on linen, which is they have a scale marked around the base. Today they are often used instead of louopes. They come in all sizes from 15mm to 90mm.
Those listed with a 'quantity' show the quantity we have in stock, these are part of a one-off job lot, no more are available once they are sold. Those not listed with a 'quantity' are regular lines, if we happen to be out of stock this month, we can get more next month.
Read all about magnification and how to choose a magnifier, click here.