This marking would be challenging to fake (although I do dislike white fill)
The Korean letters and serial number would be relatively hard to fake, especially without evidence of scrubbing the receiver to eliminate another country's serial number.
The Korean letters and serial numbers amidst deep pitting (jungle patina) would be hard to fake.
...you sure you aren't from Missouri
As a matter of fact, I was bred and born in Florissant, North St. Louis County, St. Louis suburb in Misery errr....I mean Missouri. Lived there until I was old enough to skedaddle. Then lived in Texas, Louisiana, Georgia, Montana, Wyoming, Washington St, North Dakota, Colorado, and finally ended up stuck here in Iowa on a small hobby farm. So, yeah, I question everything.
Nevertheless, I do agree with you. Although it would be very hard to fake those stamps and the rest of it's condition, it's not impossible. Just wouldn't be very cost effective.
My imagination (about how it spent it's mechanical life) isn't great enough to find that much value in it. Obviously that's just my opinion. It might be different had I been shipped to Vietnam when I enlisted but I wasn't.