1903 Barrel with "A" under date

Need help in the meaning of the letter A that is underneath the barrel marked as follows:
S A
(flaming bomb)
05
A

I appears to be a 30-03 converted to 30-06 but have yet to know for sure what the "A" means.

Any ideas?

The "A" was stamped on the barrels of rifles that passed through inspection, if the barrel was "acceptable", or still serviceable.

Adding to what Mike D stated. Rifles turned into Springfield Armory for overhaul prior to WWI were inspected upon receipt. If the barrel was deemed to be sufficiently serviceable for re-issue, an "A" was stamped beneath the barrel date.

Hope this helps. Happy New Year!

J.B.