On old sectional charts, what was the purpose of the domestic ADIZ regions and what was the difference between a "Defense Area" and an "Open Area"?
Originally they were zones to protect Americas nuclear testing areas and other key military interests. The Albuquerque area was likely there to protect Los Alamos Laboratory and other related nuclear assets of the era.
This report prepared for congress (on a slightly different topic) has some notes on this in the &quot;History&quot; section.
They explain an ADIZ as:

An ADIZ is a designated area of airspace over land or water within
which a country requires the immediate and positive identification,
location, and air traffic control of aircraft in the interest of the
country’s national security.

Which does not necessarily limit it to border monitoring. The modern ADIZ is large a border related zone but that was not always the case.

In 1948, the U.S. Air Force designated several offshore areas as
“active defense areas” or “defense zones.”3  From these, the first
ADIZs in the world were established in 1950, under an Executive Order
directing the Secretary of Commerce to exercise security control over
aircraft.4 Various defense zones were redesignated as ADIZs.5  The
present-day U.S. ADIZs, including the contiguous U.S. ADIZ, the
Alaska ADIZ, the Guam ADIZ, and the Hawaii ADIZ, are codified in Title
14, Part 99, of the Code of Federal Regulations, along with the
procedural requirements for flights operating in these designated
areas. They are predominantly located over water and typically do not
extend to the shore, leaving a narrow strip of sovereign airspace
parallel to the coastline that is not within the ADIZ.

The citation for the above paragraph explains it better (bolded for emphisis)

At the time, U.S. ADIZs included offshore regions stretching from
Maine to North Carolina in the Atlantic, and from the U.S.-Canada
border to waters off the Baja California in the Pacific. Additionally,
several over-land areas were designated as ADIZs, mostly along the
west coast and south of the U.S.-Canada border, but also above
interior lands around Albuquerque, NM, and Knoxville, TN, as part of
national security efforts to protect key nuclear research facilities
in these regions. The over-land ADIZs no longer exist. For a time,
portions of the special flight rules area (SFRA) around Washington,
DC, established after the 9/11 attacks in 2001, were designated as an
ADIZ, but this airspace is no longer referred to as such.


This map just pre dates the deployment of the Distant Early Warning System in the north and various Nike Missile Sites that were put on either coast but you can see how the American costal strategy was shaping up in a post WWII country which is interesting.
As radar became more prevalent at local facilities (think class B/C/D approach control as well as regional centers) the need for specialized interior radar was far less and those duties could be off loaded to the local facilities