The original idea of the 747 Combi came from Sabena in 1973-1974, when the oil crisis followed the October War. They wanted to reduce the seating on the BRU-JFK flights (and BRU-FIH) and replace the space for cargo. The two 747-129 they had (OO-SGA and SGB) became the first 747 Combis, which had a SCD and the rear of the main deck was usable for cargo. Actually, the seats-toilets-galleys of cabin D and cabin E were mounted on pallets "à la" QC permitting quick change from an all passenger to combi configuration.
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PanAm followed-up with the idea, quite a few 747-121 were converted to what we called CRAF airplanes (Civil Reserve Air Fleet) by the DOD and could have been used for wartime cargo, or combi configuration. Some of the Singapore 747-212B-SCD acquired by PanAm were also CRAF airplanes.
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The FAA never permitted Combis (with "aft cargo") to be operated in a mixed passenger/cargo configuration. Never were certificated as such. The only mixed passenger/cargo operation required cargo to be forward of the passengers. Only two US airlines had 747s certificated as combis by the FAA, these were the three TransAmerica 747-271C and the three World Airways 747-273C. Fact is both types had a NOSE CARGO DOOR (and only the 747-271C had a side cargo door as well).
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The problem with such airplanes, yet operated in full passenger configuration, is the extra weight of the SCD and the cargo floor. In the day and age of weight savings, having a 747 with an extra 6,000 kg basic operating weight is not pleasing the airlines. The only value to a combi comes when the 747 is later sold to another operator, and converted to a full cargo configuration, such as the 747-200SF-SCD are.
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Some 747 trivia... If FAA did not approve the "Combis" - did you know that the FAA never approved the 747-300 either...? A New York operator (Trans International Express) attempted in 1999 to operate JFK-TLV with ex-Singapore 747-312 (passenger) which would have required initial certification of the 300 type by the FAA - for an airplane that was 15 years of age.
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Yes, there are or were 707, 727, 737 or DC8 planes operated as combi by US operators, but all had cargo in front of the passenger seats. That configuration also required additional emergency exits in front of the passenger cabin (see 707C and DC8F), i.e. emergency exits installed just aft of wing on the 707C.
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With PanAm, I flew 727-100QCs in the combi configuration, we had 2 pallets in the forward cabin, the mid-aft cabin was occupied by galley and some 60 passenger seats. I also flew 707 combis, where we had 3 to 6 pallets, all located forward of passengers.
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Happy contrails