My GP has a 1899 cartouche.

Nope, just thinking about it.  Eventually I'll hit the documents but, until then, it doesn't hurt to work the noggin.  If we don't think we get nowhere. 

The GPs were made after '03 production was well underway.
Production of Krag receivers was limited, if any, at that time.
The GPs are serial blocked.  Nice grouping really.
The serials are end of production.
Guns in and near that range show up as pristine 1903 cartouched rifles.  Normal ones.

ergo, they didn't make additional receivers for GPs.

There would be three sources of stocks:
1)  New, unmarked, replacements on hand.
2)  Nice new guns sitting in storage.
3)  Returned guns.

ergo, they're "rebuilt new" late 1898s.  Grab a bunch of never issued 1898 rifles and break out the Dremel.

All of which leads me to believe there should be some with 1903 cartouches.  Hence the "rubber hits the road" post.  Lack of said cartouches would be a conundrum.

Then again I've not poked at them or given them much thought.  Pearce assembled a table but, this is a bummer, it only has numbers.  No cartouches.  That would have been helpful.

So they're either a bunch of late spare receivers or they're rebuilt .30 rifles.  Logic and observation should settle that.

118 rifles in a block spanning 1877 guns.  While "2000" seems like a lot it isn't.  Pretty tight range.

So cartouches are interesting.  1899?  That's the kind of thing that works it.