Galvanised round steel tube 15NB / 1/2" - 21.3mm outer diameter (OD) with a wall thickness of 2.3mm. Suitable for 'T' sized fittings.
Each fitting has various different sizes (A, B, C, D, E, T). First select which size tube or pipe you want to use, then look at the corresponding size in the table shown below. E.g. You want to use 48.3mm OD tube therefore you need Size D fittings.