From the Centerfire posts. R would be for November; D is for 1935, 1957 and 1983. But you said early model.
Month Codes: [first letter]
B - L - A - C - K - P - O - W - D - E - R - X
1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12
Year:______Code: [second (and third*) letters]
1935_______ D
1957_______ D
1983_______ D