From Computer History Wiki
RADIX-50 is a coding system much used by DEC, particularly in PDP-11's, to hold alphanumeric data in a fairly space-minimal form. It could hold 3 characters in a single 16-bit word. Individual characters were not stored in separate bit fields within the word; rather, a multiplication and addition process was used to combine them.
The character set supported included the characters ' ' (encoded as 0), '$' (27.), '.' (28.), and '_' (29.; sometimes replaced by '%'), the upper-case letters (1-26.), and the decimal digits (30.-39.). 40. different values in total, or 050 - hence the name. To combine the three characters, the first was multiplied by 050, added to the second, and the result multiplied by 050 again, to which was added the third.