Buckram, Fairchild: Dictionary of Textiles, on sets out what 'buckram' means:
A plain-weave, coarse, open fabric heavily sized and used principally as stiffener which is placed between the lining and surface cloth of the garment to give it shape or form. Also used for hat shapes, bookbinding, etc. Made with cotton, linen, hemp, hair, etc., CCE v. Susma Textile (P.) Ltd., (2004) 5 SCC 746 (750): AIR 2004 SC 3549. (Central Excise Tariff Act, 1985, Heading 52.06 or 59.01)