EKU

EKU stands for earliest known use of a stamp or cancel. This is similar to EDU, earliest documented use, where the item has been recorded as confirmed.

Even with well-used early stamps, official post office dates of release do not always coincide with actual use. In some cases, release date records have been lost. EKU can be used to at least bracket the dates of use of undated used stamps and cancels on cover, for example.

EKU, though found recorded on certificates, can be a moving target, with new discoveries even today.