These are Nylon 6/6 glass filled bobbins designed for EI63 laminated core, with a window 0.625" x 0.75".
Nylon 6/6 is very easy to modify with fillers, fibers, internal lubricants, and impact modifiers. Glass Filled nylon Variants of this nylon exhibit increased structural and impact strength, and rigidity. The addition of glass fibers to nylon 6/6 increases tensile strength, stiffness, compressive strength, and a lower thermal expansion coefficient over conventional unfilled grades. Glass filled nylons offer better strength than general purpose nylon.