New Guardian Board Back C4 Window Envelope 130gsm Manilla Peel and Seal. Pack of 125. These sturdy, durable New Guardian board back envelopes feature heavyweight 130gsm paper and are printed with 'Please do not bend' for mailing certificates, photos, artwork and more. Suitable for A4 documents, the C4 manilla envelopes also feature a strong Power-Tac peel and seal closure and convenient address window measuring 40 x 100mm. This pack contains 125 manilla envelopes.