I have found it difficult to identify with certainty, on U.S. Krags, if a surviving finish is original, an armory/arsenal refinish, or a worn armory/arsenal refinish.
(Commercial refinishes are normally pretty obvious).

I suspect most Krags have undergone trips back to Springfield Armory or other arsenals for inspection, repair, and refinishing.

I have read on several occasions about examining the interior of the magazine and looking for a "diagonal line or stain" as evidence that a Krag barreled-action was "re-blued" and incompletely dipped in a solution.

I am skeptical of this. I have not noticed this diagonal line (or I am not looking in the correct spot).

I have noticed that all my model 1898 and model 1899 Krag actions have a slight diagonal 'ridge' or corner that runs across the bottom-front of the magazine floor.
This appears to be the result of a machining operation. (This line is not present on my two model 1896 Krag actions, which I believe to be Armory 'blued/rebuilds').

Also, visible in the magazine and other protected areas, on some Krag receivers are marbled patterns of casehardened colors.

The red arrows in the first attached picture point to the diagonal 'machine mark', I have found on eight model 1898 & model 1899 actions, that I inspected during the past two days.

The top action also shows a lighter colored area from casehardening. The other attached photos show a variety of casehardened colors.

Attached are a couple of photos of a model 1898 Krag rifle in the 333,000 serial # range.

FWIW - I believe this Model 1898 Krag may still have its original, but worn, case-hardened metal finish.

Red arrow shows track of machine cutter. Green arrow indicates 'case colors'.

the krags went through IRAN numerous times. More times than we generally realize. Originaly the receivers were case hardened, Once case hardened it can not be done again. When the were IRAN'd they received bluing and there is indeed a spot at the inside of the lunchbox/receiver, generally diagonal, that the bluing missed, presumably because of how that part was held in the bath (my presumption). I will post a photo if my 1898 that was IRAN'd when I have a chance.

Thank you for the explanation. On my two krags I guess the rebluing  seems to have wore off the case hardened reciever. The mottled colors are visible again.  On the inside there is the area missing the bluing. Maybe it's just the way the bluing reacted with the case hardened steel verse's the untreated barrel steel.

Those lines are not smooth the way you would think if they were dipped in a bluing solution. Could they be from case hardening pack not completely filling the box? If so the are not completely cased may take the bluing differently?
In the ancient past case hardening was done by packing and sealing clay jars full of the carbon bearing material with the item being hardened. The jar was expendable and broken open when the process was finished. Any reading material available on how SA did this to the Krags. It has already been said on this forum that surfaces than needed to be extra hard and wear resistant were painted with cyanide during casing, but no mention was made of the other details.

Reviewing earlier posts and Mark's mention of bluing sand fits the irregular lines very well, if it settled or the receiver was not completely filled. Maybe someone can elaborate on the process as I have never seen it described.

For those that have a hard time reading fuzzy text on a computer screen the answer is cast iron boxes.
Thanks BR.

Still interested in bluing sand. Is that just plain sand held at temperatures to get a blue oxide, or is the sand infused with something to get a blue at a lower temperatures?

For anyone wondering why Krags would be difficult or impossible to build today, the article says 120 different machining operations were involved. Each one necessary and carrying the risk of scrapping the receiver.

Three photos taken at Springfield Armory - Hill Shops - 'Hardening Room'. I believe the pictures show: hardening of 1903 Springfield receivers-in iron boxes (1), hardening of 1903 bolts in tray (2), and possibly installation of extractor-ring on 1903 bolt (3).

I am sure Mark's post is a typo. "bluing and"not sand