I did some research with google and have found the following--
In relation to the requirement for attitude indicators for cloud flying in powered light aircraft, the answer appears to be "1956".
See: (quoting)
GENERAL OPERATION RULES
ADDITIONAL INSTRUMENTS FOR IFR OPERATIONS
This amendment includes a substantive change which requires an artificial-horizon and a directional gyro as additional instruments for IFR flight operations. This additional equipment is made mandatory because flight tests on small aircraft have shown that safe instrument flight without such equipment is extremely difficult under rough air conditions.
Interested persons have been afforded an opportunity to participate 1/ in the making of this amendment, and due consideration has been given to all relevant matter presented.
In consideration of the foregoing, the Civil Aeronautics Board hereby amends Part 43 of the Civil Air Regulations (14 CFR Part 43, as amended) effective August 1, 1956:
By amending 43.30 by adding a new note after the introductory paragraph and by adding new subparagraphs (8) and (9) to paragraph (c) to read as follows:
43.30 Instruments and equipment for NC powered aircraft or powered aircraft with standard airworthiness certificates.
NOTE: Instrument and equipment installations are required to comply with the applicable airworthiness parts of the Civil Air Regulations.
Instrument flight rules.
Gyroscopic bank and pitch indicator (artificial-horizon).
Gyroscopic direction indicator (directional gyro or equivalent).
(end quote)
Source:
http://ntl1.specialcollection.net/sc...&site=dot_cars
section 43
amendment 43-1
However, attitude indicators were already required for all "air carrier" operations. For example amendment 04b-6, dated 1947, rescinds a requirement for a "nonupsetting type gyroscopic bank and pitch indicator", noting that "Service experience with instruments of this type has not been sufficiently extensive to warrant a requirement that this type be installed to the exclusion of other types which, while not completely upsetting, have shown satisfactory service characteristics over a long period of time. It appears, therefore, that safety would be served equally well if instruments which are currently used in airplanes certificated under other airworthiness parts of the Ci vil Air Regulations are allowed to be used for the present."
(source-- link as above, section o4, amendment 04-b6)
Steve S