I'm pretty sure this ISN'T a gooseneck flare, but a simple filling can of some description.
Assuming the stamping is an AM stores reference, Section 21 is for Barrack Equipment
Subsection 21G/x covers "Hospital Equipment, excluding medical & surgical supplies" which doesn't sound right at all, more likely is 21C/x "Metalware".
Goosenecks with marked stores references are usually in Section 4 (Aerodrome Equipment), for example 4C/1273 is the reference for the regular round pattern type with the round lid.