The early Volvo AW55-50/51SN 5 speed automatic transmissions (eg Geartronic) have an issue regarding shift 'flares', occuring when shifting in and out of 3rd gear.
This is revealed by 'missing' the shift between 2th or 3rd gear, or a 'clunk' shifting from Neutral to Drive. With extremely hard shifts permanent damage could already have occured meaning this fix is too late.
The problem runs up to around 2004, later models will already contain this fix. Replacement Volvo automatic transmissions around that time will also have the fix installed.
A cheap part like this could save an expensive transmission. With later models any shift issues can usually be solved by software updates using VIDA. After this fix it is adviceable to flush.
(This fix is also known as the Servo B4 fix)