R51 is a strongly recommended feature release:

• OpenBSD sync: handle integer base out of band like ksh93 does
• Protect standard code (predefined aliases, internal code, aliases and functions in dot.mkshrc) from being overridden by aliases and, in some cases, shell functions (i.e. permit overriding but ignore it)
• Implement GNU bash’s enable for dot.mkshrc using magic aliases to redirect the builtins to external utilities; this differs from GNU bash in that enable takes precedence over functions
• Move unaliasing an identifier when defining a POSIX-style function with the same name into lksh, as compatibility kludge
• Korn shell style functions now have locally scoped shell options
• Change some ASCII-isms to be EBCDIC-aware or pluggable
• Mention lksh build instructions on manpage and website
• Overhaul signal handling; support new POSIX NSIG_MAX, add sysconf(_SC_NSIG) as a later TODO item
• Fix signal bounds (1 ≤ signum < NSIG)
• Improve manual pages, especially wrt. standards compliance
• Initial EBCDIC work for dot.mkshrc
• Add list of z/OS signals to Build.sh
• Work around the sh(1) backslash-newline problem by moving the code triggering it out of *.opt and into the consumers
• Bind another well-known ANSI Del key in the Emacs mode
• Fix ${foo/*/x} pattern checks, spotted by izabera
• Fix error output of cd function in dot.mkshrc
• read partial returns in -N and timeout cases
• Fix $LINENO inside PS1; spotted by carstenh
• Ensure correct padding of at least 2 spaces in print_columns
• Note issues with nested complex parameter expansions and follow-up bugfixes to expect
• Some language fixes in documentation; comments
• Reimplement multi-line command history (Debian #783978) + fixes
• Fix command -v for “shell reserved words”
• In dot.mkshrc make use of latest feature: local options
• Fix ""$@ to emit a word
• Change cat(1) hack to look first and not ignore builtin
• Begin porting mksh to OS/2
• Some generic minor bugfixes from OS/2 porting
• Document mknod(8) isn’t normally part of mksh(1)
• Quote arguments to : in build/test scripts as well
• Add cat(1) hack for printf(1)-as-builtin: always prefer external
• Explicitly use binary mode for any and all file I/O in stock mksh
• Use termio, not termios(4), on OS/2
• Set edchars to sane BSD defaults if any are NUL
• Implement support for PC scancodes in Vi and Emacs editing mode
• OS/2 uses ‘;’ as PATH separator plus support drive letters