I purchased this 1903 extended magazine that was recently advertised on one of the local gun boards.  I can't seem to find much information about them outside of the mag's use on Air Service and trench periscope rifles. It holds 25 rounds of 30-06 ammo, has an eagle's head in the lower corner and easily  installs on my 1903 Springfield.  Ammo cycles smoothly but it does weigh a bit more with the 20 extra rounds.  I think it looks kinda cool, however, I still gravitate more toward the '03s clean sexy unladen lines.

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That's about all the information I have on mine too.

The Air Service models were stripped of the rest of the military hardware; cut down stock, no upper handguard, bands, etc. No sling. Shooter just used the battle sights. 

Imagine being an observer in a balloon filled with hydrogen, where your real job is telegraphing information to artillery batteries.  Along comes the Hun in a maneuverable airplane flying at over 100 mph, looking for balloons as juicy targets.  The Hun has one or two machine guns, belt-fed, loaded with tracers and incendiaries, trying like crazy to ignite the hydrogen just over your head.  You get an extra 20 shots in the magazine of your 03.  Do you have a parachute?  Or will you hope to slide down the line that tethers the balloon before the damned thing falls burning upon you.