A bondage cover, as opposed to a bondage magazine, was a general-interest magazine that featured bondage imagery on its cover, usually an image of a bound and gagged woman.
Painted Covers
These were the earliest examples of bondage cover art images, and run from about 1910 (when the pulps became more common) until roughly 1975 (when men's adventure type of magazines started to disappear).
The peak era for these seem to be the 1930's with weird menace and detective pulps and the 1960's heyday of men's adventure magazines.
Photo Covers
The earlist cover with an actual photograph seems to be a March 1938 cover of Actual Detective Magazine . The earliest use of a color photo on a cover is the February 1939 issue of True magazine, with the February 1940 edition appearantly the first to feature a gag.
The peak era for these was the era from roughly 1959 until 1986, when, due to the Meese Commision , and the end of a few of the publishers of Detective (or "True Crime") magazines, the main era of the bondage cover ended, though there were a few issues of Detective Dragnet in the late 1980's and early 1990's, and a brief revival from about 1994 until 2000, though even then few and far between (unlike the late 1960's, when at least 2 such covers could be seen monthly.
Also, the use of over-the-mouth gags was common enough that the slang term "Detective gag" is used for it.
External Links
[1]Detective Magazine.com--shows a different cover daily, and has monthly galleries.
[2]The Magazine Rack--features hundreds of Dewtective Magazine cover scans (mainly non-bondage).