Note: - Always fit this alternative wheel cylinder in pairs. Before TS34404 Triumph used a 0.75" (19mm) cylinder with no other alterations so as the diameter of GWC1154 is in between the two it should work fine.
Various rear wheel cylinder sizes were used on Girling braking systems from TR3 to TR6. The cylinders can be interchanged to affect front to Back brake bias. If in doubt fit what was there originally. See below alternatives.

119600: 5/8"(16mm) bore. TR3A from TS34404 to TS56376 (10" drum)
GWC1154: 0.7" (17.8mm) bore. TR4 from CT5655(wire wheels) & CT5783 (steel wheels) and all TR4A, 250, 5 and 6 (9" drum).
GWC1118: ¾" (19mm) bore (slotted piston). TR3 from TS15332 to TR3A TS34403 (10" drum)
GWC1112: ¾" (19mm) bore (flat top piston). TR3 from TS13046 to TS15331 (10" drum) & TR3A from TS56377, TR3B and TR4 up to CT5654(wire wheels) & CT5782 (steel wheels) (9"drum)
GWC1154-1 is a 7/8" cylinder with a slotted piston. This is not a TR fitment but can be used where an extra-large piston is desirable.
Use fitting kit GRSP2862 when replacing wheel cylinders
A repair kit GRSP2030 is available for 5/8" cylinders, 512351 for 0.7" cylinders and 18G9065 is available for ¾" cylinders.