The Carcano receiver was blued; same with the Argie Mausers. Bluing is a surface treatment. When you grind off that surface, it exposes the underlying steel against the dark remaining bluing. Over many years, the exposed steel will develop a brownish patina, but even then, it's easy to tell the difference between the ground area and the blued OEM surface. And in 1963, the underlying steel wouldn't have had quite so much time to develop that patina. To my knowledge, no one ever claimed to have seen anything on the rifle found in the TSBD that would indicate such a modification.
I think that someone familiar enough with the Argentine Mauser to mistake the TSBD Carcano for one would also be familiar with 2 of it's main identifiable characteristics. The ground off Argentine national crest and the receiver markings noting the model designation and the manufacturer information.
My take from this is the opposite of yours. I don't think anyone mistook a Carcano for a Argentine Mauser. The logical conclusion is 2 rifles recovered. The Mauser didn't fit the LN story and was ghosted.
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The majority of model '91 Argentine 7.65 Mausers imported into the US had the Argentine national crest ground off.
https://gunsinthenews.com/1891-argentine-mauser-history/"Collectors in the U.S., though, often find the national crest ground off of Argentine 1891 Mausers. This was done in the aftermath of the Chaco War of 1935, which pitted Bolivia and Paraguay against one another in a vicious albeit brief struggle for control of South America’s resource-rich Chaco Boreal. Argentina provided Paraguay with a large number of Model 1891 Mausers during the conflict in a move that jeopardized its relationship with Bolivia. The presence of unground national crests made it impossible to deny Argentina’s direct support for Paraguay, so after the war Argentina instituted a law requiring the removal of the national crest from any gun leaving the country. Although the government in Buenos Aires later dropped this requirement, by then most of the Argentine 1891 Mausers had been ground, and this accounts for why it is rare to find one with the crest intact."------------------------------
https://gunsinthenews.com/1891-argentine-mauser-history/~snip~
"All of the 1891 Argentine Mauser rifles and carbines were manufactured in Berlin first at Ludwig Loewe & Company, and after 1896 at Deutsche Waffen und Munitionsfabriken Aktien-Gesellschaft (“DWM”)." 
(l.)" A 90-degree turndown bolt handle was standard on carbines. (ctr.) Graduated out to 2,000 meters, the rear sight also folds flat. (r.) Rollmarked from Berlin and shipped to Argentina, , the 1891 Mauser would also have the Argentine national crest on the front receiver band. Whether made by Loewe or DWM, the 1891 Argentine Mausers are beautifully made firearms well-known for their spectacular receiver markings. In addition to the model designation and the manufacturer information,"~snip~
