American Airlines 727-23 in flight (scanned from a postcard--hence the moire)
The 727-100 was the original variant proposed to the airlines back in the early 1960s. In the reproduced postcard above, you can see some of the identification points: oval central intake and midships galley door (just discernible between the words "American" and "Airlines". Note also that the early 727 deliveries had a lot less painted areas than we are used to now on newer airliners.
Most of the 727-100s still flying have been converted to pure freighters; see the freighter conversion page for details.
Mexicana 727-64, Mexico City (collection)
This left side view shows the door and exit arrangement typical of the
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