Thank you for your link.

The edition that I seek is listed at the Library of Congress, Copyright Office, Catalog of Copyright Entries. Third Series, 1977: July-December as:
Recommended rules for design and construction of large, welded, low- pressure storage tanks. API standard 620, July 15, 1977 6th ed. Sheets 0 American Petroleum Institute; 24Jul77; A883188.

If there is a 1978, that will help also. the 9th edition is 1996, Techstreet only shows 1956, 1963, 1966, 1970 and then 1996.

The API Catalog historical page lists 1st ed as 1956, 10th ed as 2002

The IHS page on your link appears to be 6th edition with Rev2 for 1978.
My drawings for the tank in question state that it is built to 6th Ed. 1977, I do not know what was changed in the Rev.2

If anyone has the 1978, at least it will be closer than any of the newer issues that I have.