#u536 Pre-First Day Cover

Hi everyone,

I handle a pretty massive volume of inventory daily, mostly sorting through historical covers and postal stationery, and I occasionally run into things that make me scratch my head. I'm hoping some of the experts here can weigh in on this one.

I located multiple #U536 4c red violet stationery envelopes postmarked JUL 22 / 1958.

According to the catalog, the official First Day of Issue for this Die 1 stationery was July 31, 1958, out of Montpelier, VT. However, the postmark on my cover is very clearly Los Angeles, STA. NO. 55 / JUL 22 / 1958.

That puts this postmark 9 full days before the official release.

Does anyone have experience with this specific pre-first day cancellation?
I appreciate any insights you might have!