These are coupling dampers by Stabilus of Germany the world market leader in gas springs and dampers.
Fit Bradley Doublelock current HU3HE without extension EH27 & 3200S Equivalent to KIT260
2700kg maximum gross weight.
Supplied with replacement 12mm bolt and nuts as required.
These are OE quality at a fraction of the price
OE marked 5169

Before you replace the damper itself, you should always check the brake adjustments. Sometimes poorly adjusted brake shoes and/or the brake linkage can cause the same symptoms as a failed damper.  Travelling to the bump stop constantly can also cause a damper to fail prematurely.  If you are in any doubt get a qualified trailer technician to fit it for you.