All objects from Predynastic graves from el-Mahasna were numbered with the prefix 'H', folllowed by the tomb number. Occasionally, a captial letter after the number was used to distinguish individual objects from the same tomb, in this case pottery vessel D from tomb 135 at el-Mahasna
Links
[1] https://egyptartefacts.griffith.ox.ac.uk/excavations/1908-09-el-mahasna