Harry Bloomfield wrote:

As near as I could see, what was dropped into the ground from cabinet to
every house, was a semi rigid and yellow coloured.

Just a hollow tube to allow fibre to blown in later. Given how many
tubes were bundled together into the minitrench, it was amusing to see
how they checked which was which ... one chap walked up the road with a
trolley containing a generator and a compressor and some hose, squirted
air into the stopcock for each house, and then walkie-talkied to his
mate to label which tube corresponds to that house at the cabinet end.