Summaries of the Putnam exam have appeared annually in the American Mathematical
Monthly in the following issues. (Other periodicals may carry these as well.)
Note that old issues of the Monthly are available online to subscribing
institutions through the JSTOR service; see
     http://www.jstor.org/journals/00029890.html

42 may
46 oct
47 aug-sep
48 dec
49 aug-sep
50 aug-sep
51 aug-sep
52 oct
53 oct
54 oct
55 oct
57 jan
57 nov
61 jan
61 aug-sep
62 oct
63,64,65,66,67,68 aug-sep
69 oct
70,71 aug-sep
73 feb
73 nev
74 dec
75,76 nov
78 jan
79 mar
79 nov
80,81 oct
82 nov
83,84,85,86,87,88,89 oct
91 apr
91,92,93,94,95,96,97,98 oct
99 nov
00 oct