North Korea Stamp 150 And Dating Questions

I have this Sc150 "Departure of Chinese People's Volunteers" that has a postmark from May 27th, 1958. However, Scott lists this as being from September 10th of the same year. I don't believe it's a forged postmark because any pm decreases the value dramatically for this pair. Maybe the imperf variety was issued a bit prior to Sept. 10th?

Also, how does Scott get their information from such a place. Perhaps the information they were given was not so reliable?

Found something interesting in the catalogue today:




It's a bit cryptic, but apparently the people at Scott referenced some kind of connection between the 150 and an issuance that came right before it, also tying them together with the same title; "Departure of Chinese People's Volunteers".

Furthermore, the handstamp on this pair matches the same date as the release of the one that came prior.

Could I assume that this handstamp shows it to be a first day use, and that the imperf variety WAS released before the 150's recorded date?