Can anyone help explain this marking "2500"

[LudemanM]

On a small subset of the covers from internees at the Estacion Migratoria Internment Camp at Perote, VER, Mexico, the covers have the large black "2500" hand-stamped in black ink.. This marking has only appeared (so far, anyway) on covers mailed from Mexico between early May to early July, 1944, and all of the covers were censored in Berlin while in route to the International Red Cross in Geneva.

I've not been able to find a description of this marking and its purpose, and hope that a Forum reader can help. Here is an example.

MX_CEN_VER_Perote1944-06-22)(Est Migratoria)(MX Label 505 (US 12086 German)(Rolf Warnholz)(Harlos#37-6457)both.jpg