BO3 1926 Argentina

When the LU prefix is added, DZ4 became LU4DZ.
The first letter, in this case D is the Providence or district ( D=Provincia de bsas) so the first letter is the providence not the number.
So your B cards, BO3, BO6 are both from City of Buenos Aires (BO3 is Enrique Supiciche, Buenos Aires)

EA5BM Collection
Info courtesy of Patrick Rigg