Being the MIT partisan,  I'd add Whirlwind although some might question whether its "operational date" falls within the 1950 limit.  

Debbie Douglas

On Nov 14, 2012, at 10:04 PM, David Hemmendinger wrote:

The other Bell Labs computers.  From Paul Ceruzzi's _Reckoners_:
              Model II   Model IV      Model V (two copies)
Date completed  7-1943     3-1945       12-1946, 8-1947
Also: Model I, 1939; Model VI, 1949

The Harvard Mark II, III.

The Pilot ACE (UK) ran in May, 1950 according to alanturing.net and was
demonstrated in Dec, 1950, according to Wikipedia.

 David Hemmendinger                           hemmendd@union.edu
 Professor Emeritus            http://athena.union.edu/~hemmendd
 Computer Science Dept.                          +1 518 346 4489
 Union College, Schenectady, NY 12308       FAX: +1 518 388 6789

Are there any computers missing from this list below of computers that were
built from 1940 to 1950?

Year    Computer        Country
      Name
1941    Z3                Germany
1942    ABC               U.S.
1943    Colossus          U.K
1944    Harvard Mark 1    U.S.
1944    Bell Labs Model 3 U.S.          
1945    Z4                Germany       
1946    ENIAC             U.S.          
1947    SSEC              U.S.
1948    SSEM              U.K.
1949    BINAC             U.S.
1949    EDSAC             U.K.          
1949    CSIRC             Australia     
1949    EDVAC             U.S.
1949    MADM              U.K.
1950    MESM              Ukraine
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Deborah G. Douglas, PhD • Curator of Science and Technology, MIT Museum, Room N51-209 • 265 Massachusetts Avenue • Cambridge, MA 02139-4307 • http://web.mit.edu/museum  •  http://museum.mit.edu/150 • ddouglas@mit.edu •  617-253-1766 phone  •  617-253-8994 fax