As a newly-minted sprog in 1964, one of my first liney tasks was to re-charge the crew and pax oxygen systems in a couple of Hastings visiting Lyneham.
IIRC, the pax oxygen points were under the floor in the main cabin. Trying to access them, I found traces of very black gunge around the fuselage stringers. Reporting this to Chiefy, he informed me that it was the remains of the coal cargoes, transported on the Berlin Airlift.