Answer: National Security Act of 1947

One of our documents librarians with access to United States Statutes at Large on CD-ROM got the following list of changes:

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National Security Act of 1947 (61 Stat 495)

Amended:
1949 - National Security Act Amendments of 1949 (63 Stat 578)
*1949 - To Amend the National Security Act of 1947 (63 Stat 30)
*1950 - To Amend the Selective Service Act of 1948 (64 Stat 828)
*1951 - Air Force Organization Act of 1951 (65 Stat 333)
*1951 - Mutual Security Act of 1951 (65 Stat 378)
*1952 - To fix the personnel strength of the United States Marine Corps (66 Stat 282)
*1953 - To Amend the National Security Act of 1947 (67 Stat 19)
*1954 - To provide for two additional Assistant Secretaries of the Army, Navy, and Air Force, respectively (68 Stat 650)
*1954 - To amend various statutes and certain titles of the United States Code (68 Stat 1228, 68 Stat 1244)
*1956 - Enacting Titles 10 & 32 of the United States Code (70A Stat 629/PL 84-1028)
1958 - Department of Defense Reorganization Act of 1958 (72 Stat 514, 72 Stat 516, 72 Stat 518, 72 Stat 520, 72 Stat 521/PL 85-599)
*1964 - Government Employees Salary Reform Act of 1964 (78 Stat 425/PL 88-426)
*1980 – Intelligence Authorization Act for Fiscal Year 1981 (94 Stat 1981/PL 96-450)
*1981 – Intelligence Authorization Act for Fiscal Year 1982 (95 Stat 1153/PL 97-89)
1982 – Intelligence Identities Protection Act of 1982 (96 Stat 122/PL 97-200)
*1983 – Intelligence Authorization Act for Fiscal Year 1984 (97 Stat 1477/ PL 98-215)
1984 – Central Intelligence Information Act (98 Stat 2209/PL 98-477)
*1985 – Intelligence Authorization Act for Fiscal Year 1986 (PL 99-169/99 Stat 1004)
*1986 – Goldwater-Nichols Department of Defense Reorganization Act of 1986 (100 Stat 1074/PL 99-433)
*1986 – Making Appropriations for Agriculture, Rural Development, and Related Agencies (100 Stat 1783-125/PL 99-500)
*1986 – Intelligence Authorization Act for Fiscal Year 1987 (100 Stat 3203/PL 99-569)
*1986 – Making Appropriations for Agriculture, Rural Development, and Related Agencies (100 Stat 3341-125/PL 99-591)
*1986 – National Defense Authorization Act for Fiscal Year 1987 (100 Stat 3986/PL 99-661)
*1988 – Intelligence Authorization Act, Fiscal Year 1987 (102 Stat 1910/PL 100-453)
*1988 – Anti-Drug Abuse Act of 1988 (102 Stat 4182/PL 100-690)
*1991 – Intelligence Authorization Act, Fiscal Year 1991 (105 Stat 441, 105 Stat 443, 105 Stat 444, 105 Stat 445/PL 102-88)
1992 – Intelligence Authorization Act for Fiscal Year 1991 (106 Stat 3188/PL 102-496)
*1993 – National Defense Authorization Act for Fiscal Year 1994 (107 Stat 1715/PL 103-160)
*1993 – Intelligence Authorization Act for Fiscal Year 1994 (107 Stat 2034, 107 Stat 2036, 107 Stat 2038/PL 103-178)
1994 – Intelligence Authorization Act for Fiscal Year 1995 (108 Stat 3428, 108 Stat 3435, 108 Stat 3438/PL 103-359)
*1996 – Intelligence Authorization Act for Fiscal Year 1996 (109 Stat 964, 109 Stat 977/PL 104-93)
*1996 – National Defense Authorization Act for Fiscal Year 1996 (110 Stat 353, 110 Stat 510/PL 104-106)
*1996 – National Defense Authorization Act for Fiscal Year 1997 (110 Stat 2684, 110 Stat 2687/PL 104-201)
1996 – Intelligence Authorization Act for Fiscal Year 1997 (110 Stat 3464, 110 Stat 3466, 110 Stat 3474, 110 Stat 3481, 110 Stat 3482, 110 Stat 3483, 110 Stat 3484/PL 104-293)
*1997 – Intelligence Authorization Act for Fiscal Year 1998 (111 Stat 2251/PL 105-107)
*1997 - Nazi War Crimes Disclosure Act (112 Stat 1859/PL 105-246)
*1997 – Intelligence Authorization Act for Fiscal Year 1999 (PL 105-272)
*1998 – International Religious Freedom Act of 1998 (112 Stat 2787/PL 105-292)
*1999 – National Defense Authorization Act for Fiscal Year 2000 (133 Stat 717/PL 106-65)
*1999 - Intelligence Authorization Act for Fiscal Year 2000 (113 Stat 1606/PL 106-120)
*2000 - Intelligence Authorization Act for Fiscal Year 2001 (114 Stat 2831/PL 106-567)
Intelligence
*2001 - Intelligence Authorization Act for Fiscal Year 2002 (115 STAT 1397/PL 107-108)
*2001 – USA PATRIOT ACT (115 STAT. 387/PL107-56)
*2002 – Homeland Security Act of 2002 (116 STAT. 2149/PL 107-296)
2002 – Intelligence Authorization Act for Fiscal Year 2003 (116 STAT 2390/PL 107-306)
2003 – Intelligence Authorization Act for Fiscal Year 2004 (117 STAT. 2603/PL 108-177)
*2004 – Ronald W. Reagan National Defense Authorization Act for Fiscal Year 2005 (118 STAT. 2064/PL 108-375)
2004 - Intelligence Reform and Terrorism Prevention Act of 2004 (118 STAT. 3643/PL 108-458)
2004 – Intelligence Authorization Act for Fiscal Year 2005 (118 STAT. 3944/PL 108-487)
2005 - Emergency Supplemental Appropriations Act for Defense, the Global War on Terror, and Tsunami Relief, 2005 (119 STAT. 244/PL 109-19)
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To check this in more detail, you can access the Act on the web.
"Search the US code" gets 157 hits for the search: "national security act" and amendment. These all appear to be from the @2004 and later, but next to number 92 in the first page of those 192 hits it says "Table of Popular Names: National Security Act of 1947". Click on that title to get full legal citation:
-CITE- TABLE OF POPULAR NAMES 01/06/03
National Security Act of 1947
-COD-
July 26, 1947, ch. 343, 61 Stat. 495 (50 U.S.C. 401 et seq.)Short title, see 50 U.S.C. 401 note

Under that one it has the amendments of 1949:
-CITE- TABLE OF POPULAR NAMES 01/06/03
National Security Act Amendments of 1949
-COD- Aug. 10, 1949, ch. 412, 63 Stat. 578
Short title, see 50 U.S.C. 401 note

Next, from the Download page
Follow the "Title 50 -War and National Defense..." link to "Title 50 - Complete Title". You can use CTRL-F to search 343 to get to references to section 343, and then to 50 USC Secs. 343 to 347. It could well be a big hunt in this huge 3.3564 MB text file, but it appears to be the full story of what happened to the original law over the years.

You may also want to search for books on National Security, or possibly magazine articles in Masterfile Premier (available in most public libraries), or Academic Search Premier (avail in most academic libraries). If it's tough to get to libraries, you can try web searches like: "national security act" history site:edu , or "national security act" history site:gov

Jim Miller
University of Maryland
jmiller2 {at} umd.edu