Judging by the age and style of its neon sign, this has been in use as an apartment building since the early part of the 1900's. When I go past it again I'll see if they have a plaque posted with more historical information. It looks like a place with a lot of personality and it is in the heart of downtown Tucson - corner of 13th Street & 6th Avenue.
San Carlos Apartments
71 E 13th St