Nicely made bladder conversion, M54x1 thread and to suit 52mm internal diameter nitrogen reservoir. Comes with a 50x105 which seemed slightly stretched so I used a Kayaba bladder 52x104. Fitted with a Schrader valve which easy to fill.
Make sure your reservoir it a two piece construction before buying as newer shocks use a one piece design which you'll need a different bladder kit for.
Fitting a bladder should make your WP shock slightly plusher on low speed bumps and should reduce heat build up from nitrogen piston oring friction and probably easier to bleed the shock.