You can get the same effect by combining a good thick down coat with an "elephants foot,"
Incorrect. The ABOMINABLE SLEEPING BAG converted the bottom half to individual legs with feet via a zipper design. Likewise, the top half converted to individual arms with the same zipper design. All zipped up, it is a large mummey bag with thick loft.
The bag was intended for extreme cold. The smallest I have been able to roll them up fit in a 6 gallon dry wall bucket.