Supplement III: Eldritch Wizardry was published in 1976. Interestingly there are things in this book that seem connected to the FFC:

Three items from the magical items chapter stand out:

The invulnerable Coat of Arm:
It has been speculated that this item was created by Gary as a tribute to David Arneson. OTOH, it is interesting that Arn appeared as a realm in BTPTBD. Was "Arn" more than just a reference to Dave?

Throne of the Gods:
This item is referenced in the FFC. Not saying Dave created it, but it certainly appeared in his campaign. It is possible it was added after Dave got the supplement, or Dave got to know about it before its publication. The Throne is supposedly created by an ancient race. Not the actual Gods from City of the Gods?

Orb of Dragonkind:
Although it is never mentioned by name, this item maps very well with an artifact mentioned in the FFC found on the Island of Father Dragon. See comments on the Throne of the Gods above.

-Havard