New Guardian Heavyweight Manilla Board Backed Envelopes. Send documents with confidence in these high quality manilla board backed envelopes which provide a combination of good looks and strength. Exceptionally strong 130g/m ribbed manilla paper across the whole range. Subtle ribbed texture and distinctive sheen provides an attractive finish. Backed with a sturdy board to keep contents flat and protected. Printed 'Please do not bend' instructing considerate handling. Peel and Seal gumming for an exceptionally strong seal.