this is that very special bike , build of bits and bobs ... with a battery
riding pillion .
that pipe stub could very well be a piece from a lamp or Chandelier .
lamps use 1/8" IPS thread ... an extremely common but less-accounted for thread form .
most us have a piece of this pipe left-over from a light -fixture , in a junk drawer ...
... a normal pipe cap , in lamp language , is called a finial
IPS thread is ( Iron pipe straight) ... the same as british non-tapered thread ...
somewhere in the past they have a common ancestor . ( early town gas pipe ? )