Page "XMODEM" Paragraph 35

XMODEM-1K was originally one of the many improvements to XMODEM introduced by Chuck Forsberg in his YMODEM protocol.

Forsberg suggested that the various improvements were optional, expecting software authors to implement as many of them as possible.

Instead they generally implemented the bare minimum, leading to a profusion of semi-compatible implementations, and eventually, the splitting out of the name " YMODEM " into " XMODEM-1K " and a variety of YMODEMs.

Thus XMODEM-1K actually post-dates YMODEM, but remained fairly common anyway.