Nic, it's actually a very very good question.
In general, it is true. When a year range is given on a stamp, the last year is often the same as the year the stamp was issued. And in the example you provided, the stamp was in fact issued on August 15, 1934 to commemorate the 20th anniversary of the Czechoslovakian Legion which fought in World War I.
There are a few exceptions, of course. I have seen a least one example of these types of stamps issued one year before or one year after. A bit more common are some cases in which the stamp was issued on a 5/10... year anniversary of the last date on the stamp.

k