In the very early early 70's I used a Resin Coated (PE) paper that was ex-Government surplus, and its manufacture pre-dated the release of similar papers on the commercial market.
This early RC (PE) paper was cut from rolls, and available from AW Young, Marston & Heard, & Harringey Photographic Supplies. These three companies specialised in selling ex Military of Defence (UK) surplus and outdated film, paper, chemicals and equipment. The films & papers were all of manufactured in the UK.
Early RC (PE) papers were designed for rapid printing & processing of aerial images for military use.
Ian