Sloppy record keeping?  Or was there a separate division or accounting practice for equipment made specifically for the National Guard? 

By 1908, the Regular Army was pretty well on the way to being equipped with the New Springfield.  The .30-03 rifles had been recalled and modified for the 1906 cartridge.  But National Guard units armed with the Krag saved their empty brass and reloaded it.  Such gauges would be used by the unit armorer in checking empty brass before reloading. 

If Springfield was going to tool up to make such gauges, for either cartridge, why would they only make 1?  Unless it was a prototype, with production to follow after testing.