These two have exactly the same data printed on packaging: brand ownership is the same (Carozzi), the same (exactly) list of ingredients, and the same manufacturer address.
Today’s pricing in Jumbo is $1.479 x kg vs. $1.159 x kg:
Harina con polvos de hornear Selecta
$1.479 x kg
Harina con polvos de hornear Mont Blanc
$1.159 x kg
And, practically, when used, they are different. Why so? Do we have any insider in the industry?
While researching, have found this very informative article.
http://www.economiaynegocios.cl/noticias/noticias.asp?id=255631
So, here are the key dates:
On December 4, 1916, the Compañía Molinera San Cristóbal S.A. was founded in the city of Valparaíso by a group of immigrants of Scottish origin, owners of the firm Williamson, Balfour and Company.
In 1967, the English firm Williamson Balfour decided to put an end to its operations and sold the San Cristóbal mill, one of the most important in the country at that time, to a group of millers, including José Borda Aretxabala and his friend, relative and partner, Fermín Diharasarri Zugarramurdi, who together and over time would become the main shareholders of Molinera San Cristóbal S.A.
In 1982, Compañía Molinera San Cristóbal merged with Molinos Mont Blanc, practically doubling the company’s grinding capacity. This merger allowed San Cristóbal to have a size and efficiency that led it to position itself as a leader in the national milling industry.
At the beginning of the year 2000, the company manufactured its Selecta and Mont Blanc flour brands at its main plant and at the Maipú mill.
Cristóbal Borda Mingo, president of Molinera San Cristóbal, since 2006, led and managed one of the most transcendental processes in the company’s history, which consisted of the sale of all mass consumption products to Empresas Carozzi S.A. and thus dedicate itself exclusively to the manufacture of industrial flours and complete mixtures.
So what we see in supermarkets (1kg bags) is the result of this sale of mass consumption products to Empresas Carozzi S.A.
But the huge difference in pricing (Selecta vs. Mont Blanc) is still unclear to me, as of yet.
There is local and imported wheat, but the local seems to be processed mostly by the Southern mills, like Molino Kunstmann in Valdivia, Molino Villarrica, etc…
There are various types of wheat (so both the local and imported wheat can vary in price) but I fail to find on the Internet any sources that would disclose which types of wheat are used by which brand.
Very well, as no one interested, here is another case:
All of the info on the label absolutely identical (as NESTLE owns Leche Sur), but the price is different. Leche Sur is always cheaper. Why so?..
They (NESTLE) list all of their Chilean processing plants on the website. In what way one brand is better? Please tell! Do we have insiders?.. 
List of fabricas:
Company history:
So, in 1993 Leche Sur was bought by Nestle.
And now, at the very same plant (“fabrica Los Angeles”) these two brands are made:
Not that we are not interested. And yes, would be good to know if the premium on one is worth anything.
I have pointed out before that cheaper store brands for certain foodstuffs especially condiments are products from a “known” household brand.
Regarding the two pairs noted, my observations show that the two “different” brands of NESTLE owned condensed milk are pure corporate BS, they are the very same product packaged on the very same line of the same plant. In the past, the LECHE SUR was produced on their own plant, but the historical records on the Internet are not easy to find. Also, anyone remembers the past design of the LECHE SUR showing a mapuche (the image was removed a couple of years ago)?
The two flours however differ in reality, they have not the same texture, color, and property when used for baking, but the source of wheat (local vs. imported) and the type of it are (intentionally or not) not disclosed to the public.
Was searching for that old design, and finally found on Twitter, here it goes:
