These are equivalent to mail lite/feather-post size K - the internal measurements of each envelope are 340mm x 445mm. Classed by Royal Mail as 'Packet' size. 500 x Size K (7) Jiffy Aircraft Padded Bubble Envelopes.
500 x Jiffy Airkraft Size 7 (K) Padded Envelopes 340x445mm[5055502316369]