At FredC's request, I am posting a chart (recording the basics) on the twelve U.S. Krag Bolts, that I measured using his fixture.
(Fred's device precisely measures distance from recessed bolt-face to rear of locking-lug).

Eleven of my Bolts are model 1898 bolts. One is a "New Old Stock" late-style model 1896 Bolt (from Grandpa's Gun Parts).

Two of the 1898 bolts came to me in model 1896 Krag rifles that were refurbished in the early 1900's. Both bolts closed on my 'Field Gauge'.
  (I switched these two Bolts between the model 1896 barreled-actions. One Bolt 'worked' and no longer closed on the Gauge. The other action required a 'New Old Stock' model 1896 Bolt).

Six model 1898 Bolts are in the Krag rifle or carbine they arrived in. The bolts do not close on my .073" Field Gauge. (Fired Cartridge cases will interchange in the chambers of these Krags).

Four Bolt/Receiver combinations are known to be replacements. They also pass the Field Gauge test. (Oddly, the two bolts with the worst dimensions still pass in the receivers, I have put them in.

Two Bolts, (one NOS and the other showing some wear), are in my spare parts box.

Here are my results. I do not see a pattern to the markings.

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