It's a 1961 by the Winchester records. The last serial number made in 1960 was 504257, the last serial number made in 1961 was 545446. So that makes your a 1961. 
That's what I came up with too.
RECORDS AT THE FACTORY INDICATE THE FOLLOWINO SERIAL NUMBERS WERE
ASSIONED TO GUNS AT THE END OF THE CALENDAR YEAR.
MODEL 70
1935 - 1 TO 19 64 - 700000 TO 740599
36 - 2238 65 - 809177
37 - 11573 66 - 833795
38 - 17844 67 - 869000
39 - 23991 68 - 928908
40 - 31675 69 - 0941900
41 - 41753 70 - 0957995
42- 49206 71 - 01018991
43 - 49983 72- 01099257
44 - 49997 73 - 01128731
45 - 50921 74 - 01175000
46 - 58382 75 - 01218700
47 - 75675 76 - G1266000
48 - 101680 77- 01350000
49 - 131580 78- 01410000
50 - 173150 79 - 01447000
51 - 206625 80 - G1490709
52 - 238820 81- 01537134
53 - 282735 82 - 01632872
54 - 323530 83 - 0165688:) 55 - 361025 84 - 01728457
56 - 393595 85 - 01783276
57 - 425283 86 - 01808838
58 - 440792 87 - 01845122
59 - 465040 88 - 01893903
60 - 504257 89 - 01950701
61 - 545446 90 - 01987984
62 - 565592 91- 02037985
63 - 581471 92 - 02037985