Hi, as Pete7 says, they're great bits of kit. Here's one I fitted to a customers' trailer when I fitted a new drum and shoes etc, during a service.
The Indespension part number is HU008. You'll also need 2 x HU013, which are the plastic caps that cover the ends and protect the grease nipple and spring.
All the best
PS. Pete's also correct about the front roller swing beam upgrade, but be careful about specifying 'Warrior', as this has extra long 'L' brackets, that hold the roller arms on. You'll probably just need normal height 'L' brackets.