prevent stamp theft, some chops were used in
Since the chops are on the back of the stamps, it is certainly not identifiable on cover. I suppose it could be used to prevent theft of stamps, but once applied to a cover no one would know.
The earliest one that I have seen is on Scott 1031 the surcharge on Sun Yat-Sen issue of 1950. The illustration is dated July 6, 1950 and is on a pair of stamps. The chop reads “Taiwan Health Department”. Unfortunately, this is the only example I have at the present. Check the back of your used stamps during this period, you might just be lucky and find the complete chop on a block of four.
front of the stamps Health Department Chop
Taiwan Health Department Chop on the reverse