Registers of mass intentions as sources

Although fading in the secularized West, mass intentions are a major source of funding for priests who receive insufficient financial support from bishops in poor dioceses. Catholic priests are required to note mass stipends they have received, using books like the one pictured above. 
Earlier, they were a part of every priest's routine, as the following document from 1925 shows. It is a report on how many mass stipends a missionary group distributed in 1925. Catholics would order mass intentions, give the money to the church group in the USA, and that group would then distribute the stipends internationally. As a result, the priests who celebrated the masses had to keep a record of the masses they said. 

Such records are immensely valuable historical sources because they show where each priest was on each day, and the altar where he said mass, as explained here.