The first one is 15 centavos registered cover circulating within the Buenos Aires metropolitan area with certification mark on the back.
The second one is an interesting use of the 2 1/2 centavos, which was intended for mail for the blind, for which use I have not seen. This use, although rare, can be found, and that is a simple 5 centavos within country letter. There is an interesting exception here, with a "late train" marking on the back.
