Unlike his brothers who all joined their fathers’ regiment, Spry rebelled.
fathers' would imply that the Spry brothers had more than one father, all of whom were members of the same wedgiment.
Unless Mrs Spry was married several times, this is presumably apostrophic abuse on Percy's behalf, who perhaps intended to have written
father's. Either that or he is implying that she was the wedgimental penny-farthing....
In which case he is an absolute shower, a cad and a bounder who should be cuffed severely about the swede.