If a 51-55 arrives with a damaged threaded piston end , often the case as the seal can fail, allowing the retaining nut and collet to batter the bolt. You can cut the bolt off the piston end, drill and tap it then screw the steel adapter on. Now a HW 30 seal will fit. Walther wisely put a small tell tale on the 51-55 piston that would "ting" when the seal needed replacing. Then they would last indefinitely but if ignored the whole piston end gets battered. In my collecting days I bought several LGV pistons off Egun to put into 51-55's that needed repair. On the LGV-LGVS I usually just replace the seal with a JG but the JG LGV adapter gives more seal choices.
Eventually I will reassemble a "hybrid" 55....a LG 55 beavertail stock that Frank beautifully restored, a sleeveless 55 receiver-barrel with a LGV piston and JG seal and a JM spring. Should be lovely but has been in a box for about 2 years ...lol