These are Menzel top slides of different sizes. 51202 measures 18 x 18 mm. 51203 measures 20 x 20 mm. 51205 measures 22 x 22 mm. 51206 measures 24 x 24 mm. 51207measures 24 x 40 mm. 51208 measures 24 x 50 mm. 51209 measures 24 x 60 mm. 51210 measures 24 x 32 mm.