Aside from the first three Foundation books, which rare and Extremely Expensive, I've managed to acquire most of the Foundation universe books.
1. Triangle: Gutter Code C13 on page 516 of both the trade and SFBC issues. Both produced from the same print run, but the BCE was released as a May 1961 selection while the trade issue was published 2 June. Making this edition a "True" first edition.
2. The Robot Collection: 1st edition / 1st printing. Gutter code "Y37" with underline denoting a September 1983 printing.
3. The Robots of Dawn: 1st edition / 1st printing. Gutter code "Y35" with underline denoting a October 1983 printing
4. The Foundation Trilogy: 1st edition / 1st printing. "M44" gutter code, October 1982