Instead of envelopes, postal cards were used by many collectors to save a penny.
In the case of Irwin, California, the closure trail is documented in the "Postal Bulletin" issues:
Postal Bulletin, Issue 20103, August 28, 1958, page 4:

Postal Bulletin, Issue 20105, September 11, 1958, page 4:

Postal Bulletin, Issue 20108, September 25, 1958, page 4:

Postal Bulletin, Issue 20122, December 31, 1958, page 3:

The point still being valid that collectors could easily follow the announcements in the Postal Bulletin and send for last day covers. It is much harder today.