baseball-reference has a table indicating missing play-by-play data.

Here's the link:

http://www.baseball-reference.com/about/coverage.shtml

It was not easy to find but it's there.  Once you get to the page cruise down more than half way to:

"Play-by-Play Accounts and Box Score accounts"

Play-by-Play for all games: 1974-2012

Play-by-Play for no games: beginning of time through 1944

Games:
Year, PBP, no PBP, "percent" PBP

1973	1940	3	0.9985
1972	1857	2	0.9989
1971	1929	9	0.9954
1970	1944	0	1.0000
1969	1946	0	1.0000
1968	1602	23	0.9858
1967	1611	9	0.9944
1966	1605	10	0.9938
1965	1612	11	0.9932
1964	1623	3	0.9982
1963	1616	3	0.9981
1962	1616	5	0.9969
1961	1430	0	1.0000
1960	1236	0	1.0000
1959	1233	5	0.9960
1958	1227	8	0.9935
1957	1202	33	0.9733
1956	1189	50	0.9596
1955	1168	66	0.9465
1954	1174	62	0.9498
1953	1146	94	0.9242
1952	1088	151	0.8781
1951	1076	163	0.8684
1950	1019	219	0.8231
1949	933	307	0.7524
1948	729	508	0.5893
1947	782	461	0.6291
1946	440	802	0.3543
1945	541	689	0.4398

As indicated in the previous two posts I learned this the hard way.