The letter will likely tell you that it shipped to Springfield Armory sometime in the spring/summer of 1917.

.45 Auto and .45 ACP are one and the same, shoot away (assuming it is mechanically sound).

With a serial number that low, the grooved hammer, and the GHS stamp, I would consider it more desirable- I would put the value closer to $1000. If it were offered to me at $700 I would buy it, assuming it is in good mechanical condition.