I don't think the bogus carbine sight had the base 'ramp curvature' ground down.  Someone has simply put a 1902 (rod bayonet 1903) sight leaf on a model 1898 rifle sight base (and put two fake 'c' stamps on it).

  This was not done by Springfield Armory.  The model 1898 sights were for a higher velocity .30-40 cartridge that proved too 'hot' for the Krag.  The model 1898 sights were taken out of service.  The leafs had the eye piece re-contoured and were used on model 1902 bases and are a variant of the model 1902 sight.  The 1898 bases have the wrong curvature for the standard .30-40 cartridge.  I imagine the bases were put aside as scrap.

(p.s. The early 'rod-bayonet' Springfield sights had a short career and were replaced with the 1905 sights.  The 1903 leafs and locking knobs show up on Krag model 1902 rear sights that probably required arsenal repair.  These minor parts were interchangeable).

1.  Photo showing an 1898 rifle sight base with a 1902 leaf.  (I put this together to salvage parts on a 'cut-down shooter'). 
2.  It appears to match the bogus carbine sight 'kw64' inquired about.

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