These are types of glass microfiber Whatman filters of specified pore size, according to the related technical-commercial specifying sheets, labels, or catalogues; often currently freely available from the internet (e.g.): http://www.chimica.unipd.it/nicola.tiso/pubblica/_private/Utile/catalogo%20whatman.pdf http://www.bangkokscience.com/download/bangkokscience_com/Whatman%20Filter%20paper1.pdf