| # covers | # covers | % covers | % covers | ||
| w/Amer. | Non-Amer. | Total | w/Amer. | Non-Amer. | |
| 1936-1939 | 89 | 23 | 112 | 80.4% | 19.6% |
| 1940-1949 | 347 | 61 | 408 | 86.7% | 13.3% |
| 1950-1959 | 278 | 69 | 347 | 82.0% | 18.0% |
| Total | 714 | 153 | 867 | 82.4% | 17.6% |
| 1960-1969 | 225 | 58 | 283 | 79.5% | 20.5% |
| 1970-1979 | 95 | 11 | 106 | 89.6% | 10.4% |
| 1980-1989 | 51 | 9 | 60 | 85.0% | 15.0% |
| 1990-2000 | 54 | 7 | 61 | 88.5% | 11.5% |
| Total | 425 | 85 | 510 | 83.3% | 16.7% |