Boeing did offer the B757 with a cargo container option - no airline ever took it up. The containerised option had 7 LD
-W containers in the fwd hold, and 6 LD
-W containers in the rear hold.
I'm lead to believe that Northwest almost did take this option on their 757s to ensure speedy turnarounds, but backed out at a late stage due to technical concerns.
I know Boeing also offered containerised cargo holds on the B727-200. Not the -100 though, since their cargo doors open inwards like the B737. I've seen photos of TWA B727-200s having containers loaded, and this photo of an Eastern B727 from 1977 shows they also used the system. Take a look at the enlarged version...
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Photo © AirNikon
I'm off to see if I can find some more examples...