HCC-AX is the correct steering box for a 65-66 K code and also other V8 mustangs that had manual steering and the special handling package.
4M16C is the assembly date of the box, which decodes to:
4=1964
M=December
16=16th day of month
C=Most likely a shift code
Keep in mind that if Jan 11th is what is on your door warranty plate, it is not necessarily the actual build date. It is simply a "scheduled" build date and could be off by weeks before or after.
A mid-December '64 steering box would be perfectly acceptable on an early Jan '65 car though.