Some countries, especially in Germany and German states, began using paper registration labels on their registered mails in the 1870s or thereabouts.
In 1883, the Universal Postal Union issued a regulation that required that all member countries identify their registered letters with a paper label with the large "R" and sequence number to be applied on all mail sent to foreign destinations, which was the jurisdiction of the UPU.
The United States used these labels between Jan 1, 1883 and Jan 24, 1911.
One such example is here...
In the US, the internal regulations required that the sequence numbering restart at the beginning of each quarter. This was done at all Exchange Offices except New York, where the volume was so large that they restarted much more frequently (numbers ranged from 1 to 100,000)
In Mexico, the numbers started on July 1 and ran to June 30 of the following year until sometime in the 1920s I believe, then they changed and a new sequence was started on Jan 1st of each year.
No doubt, other countries used some sort of similar pattern. However, once the pattern is known, a collector can use the date a label was used (if on a cover), and the number of the label, to estimate the volume of registered mail from that post office for the year.