""bluing sand"? - I think possibly that is a 'typo'.

Maybe Mark's post should read: "bluing and ....""

I guess that explains why I never heard of it before. Too bad it was not a real thing as it would have been a good explanation.
I have made so many typos, that when I go back and look at my old posts I am sometimes not sure what was intended. I usually look over what was typed before hitting the Post Message button but I see what I intended to say instead of what is actually there.

BR, on your 3 photos #1 definitely look like cast iron packing boxes in the lower right.
#2 on the bolts maybe that is a tempering operation? Ovens are completely different. Looks like the bolts are just stacked in similar boxes. Look at the boxes of bolts on the floor. First ones in are upright and later ones leaning over?
#3 Must be winter time. In the summer I would have wanted that work station a long way from that hot stuff going on behind him.

Take my comments as guesses also.
Labor must have been cheap back then. My father told me about baling hay for $1.00 per day at the end of the depression. Must have been much less at the arsenal during the late 1800s.

Manufacturing has usually paid higher than other jobs. Would have expected the same at SA. Costs of everything was lower. You posted that list of prices for component parts for the Krag. Compared with today's costs they were disgustingly cheap.
Comparing apples to oranges a Krag probably cost 15 days wages, a Remington 700 today cost probably 3 days wages.
My mother used to pay 10 and then 25 cents for my haircuts when I was a kid.

BR,
Those figures sure mess with my calculations. If I am remembering correctly the cost to make one rifle or carbine was $7.50. The $2.50 guy could have bought one for 3 days labor if they were for sale.
Wowsers!