Although not normally made available to the general public, some have found their way into private hands, and they are a recognised stamp collecting speciality.
Small series of booklets were produced with large coloured numbers instead of the stamps for this purpose.
[3] They were printed in the same size and format as the definitive stamps of the UK and perforated in the same manner.
At the time of this colour change, text was also added to the centre of the labels indicating their use.
The United States has made extensive used of test stamps with many different types issued over the years.