In the letter, Leopold wrote that "the new church of Father Parhammer's orphanage will be consecrated on the Feast of the Immaculate Conception.
For this feast, Wolfgang has composed a solemn mass, an offertorium and a trumpet concerto for a boy...".
The consecration is known to have happened, as the Wienerisches Diarium reported on the service on 10 December 1768.
However, as the trumpet concerto does not appear on Leopold's list of his son's works, it is uncertain whether it ever actually existed.
The surviving manuscripts from the Kirche Mariä Geburt, now in the Wiener Priesterseminar, show no trace of the work.