1903 rifles, in addition to the original Springfield and Rock Island guns, were also made by Remington at the beginning of WWII and all saw service in the early days of the war, especially in the Pacific.
The 1903A3 was a wartime expedient gun with production shortcuts (stamped parts, receiver mounted peep sights, 2 groove barrels, etc.) to hasten manufacture and reduce cost. While I'm sure some 03A3's saw service, it seems that most were never issued due to the large number of "mint" guns sold off by DCM in the 50s and 60s. By the time large numbers of 03A3's became available, production of the M1 Garand was in full swing and, consequently, were issued to front line troops. (At least in the U.S. Army.)
"Low number" refers to those 1903 receivers that had questionable heat treating causing them to become brittle. Those made at Springfield Armory are numbered at 800,000 and below, although this is an arbitrary number selected because nobody really knows the exact number when the double heat treating process was initiated.
There is at least one documented receiver failure in a 1903 action at about 805,000 that was proven by metallurgical examination to be "brittle." Rock Island Arsenal kept more precise records and the cut off for RIA 1903's is SN 285506.
