[I have a 03A3 Mark I that has been with me for over 50 years. I would say it is 90 to 95%. Barrel date is 8-18.]
Not all original issue, as I understand your post.
The 1903-A3 is a peep-sighted WWII rifle, made by Remington & Smith-Corona.
The barrel date "8-18" indicates August 1918 - well before WWII.
Many Springfields were arsenal refinished and/or rebarreled with whatever was on hand.
Gunsmiths and others have been known to install a replacement surplus barrel for whatever reason.
An 03A3 barrel would typically be dated behind the front sight in the 40's, and be smooth in the area where the 1903's rear barrel sight would have fitted, ILO grooved or drilled for a sight base setscrew somewhere.
There was also the 1903-A1, which (since made 1930-41) also wouldn't have a 1918 barrel as the original issue.