There may have been a bit of overlap between the end of the Avenger and the start of Sunbeam production, but the Sunbeam was the replacement for the Avenger,although admittedly some moron in the accounting or marketing dept did decide to make a version with the little hairdryer motor under the bonnet.
Not true; the Sunbeam and Chysler/Talbot Avenger were available concurrently from '78 through to '82.
Sunbeams use an Avenger structure, shortened by about a foot. Many panels are interchangeable, if not the same; wings, front panel headlights, windscreen, front doors are the same as a 2 door Avenger.
There are photos of the 'Avenbeam' that Chrysler built, using the front end of an Avenger on a Sunbeam body which looks decidedly odd.
My interest in these is for the usual reason, my mum had three Avengers in a row. The full set; a Hillman, Chrysler and a Talbot. My first car was a Sunbeam Ti(wish I still had it, they're rare and worth good money), and I'm currently prepping my own Avenger for paint.
Anorak is now back on the hook!