HZ123:
I have just looked in an old B707 book of mine and there is no way that your namesake can be the 6th production model. She is a 707-138B, made her first flight on 20/3/59 and was delivered to QANTAS as c/n 17696 VH-EBA on 16/7/59.
The first production aircraft was a 707-121 c/n 17586 N708PA first flight 20/12/57 and delivered to PanAm on 30/11/58.
So, there were at least 100 707s built before they got round to HZ-123.
I seem to remember that QANTAS was one of the few airlines that bought this model. It was virtually a 320B but with the short fuselage. I was once told that they opted for this long-range version because the runway in Bahrain was only 7000 ft long and that was too short for a 320B.
Great things these old books; it's not often that I get a chance to be a complete anorak!