The U.S. aircraft register, updated daily
Special conditions

Special Conditions; Aviation Specialities, Inc. - Sikorsky Model S-55 Helicopter

Special conditions are rules the FAA writes for one design when the standard certification rules do not cover something new about it. These were for Aviation Specialities. System: other and general.

What it applies to

Applicant
Aviation Specialities
Makes
Sikorsky
Models
—
Effective
June 22, 1970
The FAA's document
On the FAA's site

Source: FAA special conditions (final) from the Dynamic Regulatory System; the Federal Register carries the same notices.