This rigid 50mm long joiner is made from strong polypropylene. It's useful to make rigid joins with push in fittings that are dead straight. Although it's possible to make joins using short pieces of 8mm OD Evabarrier 5mm Beer Line this is far more flexible and when mounting parts to a gas board or wall its will often make things much neater if you use these instead.
The 50mm length is long enough so even after pushing right into two duotight 8mm fittings on both ends you should still have a exposed length of about 3mm so you can still relatively easily take the fittings apart later.
From: Kegland Dimensions: 8mm OD x 6mm ID x 50mm long