This is a blanking / dust cap for the use with IBC tanks.  It has an S60X6 60 mm coarse thread (thread pitch is 6 mm)

This is the most common thread found on IBC tanks (80% have this thread)

Sometimes referred to as a DIN 61 Cap

Available in various pack sizes.  The more you buy, the cheaper they are......

• Buy 1 @ £3.99
• Buy 2 @ £3.00 each
• Buy 5 @ £2.60 each
• Buy 10 @ £2.00 each