You want to replace with like year cases. The factory dispatch records for a Triumph of that time would have included the engine,frame and transmission numbers. If it is an all original bike and the case is repairable then you might want to do that.But, you can use a case within the motorcycles manufactured time frame.
There are a few different variations of the the pre-unit cases...inner and outer.If your bike is a swing-arm model then the swing-arm model cases from 1954-1962 will work for you. The difference between a rigid case and a swing-arm case is easily identified. So, what year motorcycle is it?