The 29 refers to the year of excavation (1929) while the number under the line below it is the context, in this case area 1900 which is described in the excavation report as a settlement area.
Excavations at Mostagedda were in two seasons so numbering is either 28 (for 1928) or 29 (for 1929).
Links
[1] https://egyptartefacts.griffith.ox.ac.uk/excavations/1927-28-mostagedda
[2] https://egyptartefacts.griffith.ox.ac.uk/excavations/1928-29-mostagedda