According to MS-DOS 2.11 source code, MS-DOS 1.26 is an unreleased internal version of MS-DOS.
1.26 03/03/82 Directory buffers searched as a circular queue, current buffer
is searched first when possible to minimize I/O
03/03/82 STORE routine optimized to tack on partial sector tail as
full sector write when file is growing
03/09/82 Multiple I/O buffers
03/29/82 Two bugs: Delete all case resets search to start at beginning
of directory (infinite loop possible otherwise), DSKRESET
must invalidate all buffers (disk and directory).