Many of the earliest computers stored and manipulated numbers in various decimal codings rather than in pure binary. Examples include the Mark I and ENIAC, as well as some UNIVAC and IBM models.
Could any decimal-based computer handle text? If so, what encoding was used for letters and other characters?
This question asks the opposite: early computers that could only print numbers.
Really early computers like the Mark I and ENIAC didn't have enough memory to attempt to handle text; also the use-case was mostly calculations.
A number of decimal IBM computers used characters (with 5 or 6 usable bits) as the basic unit, and decimal digits were just a special usage of those characters:
The IBM 1401 computer, and its compatible successors such as the 1410 and 7010, used seven bits in memory to represent a 6-bit character plus a one-bit word mark. Decimal arithmetic was based on numbers build from those characters.
The IBM 1620 computer organized memory into two-digit cells, with four bits for the digit, and one flag bit. In alphanumeric mode, those 5 bits were used for characters. Wikipedia has a table of the encoding.
The IBM 700/7000 series used a memory consisting of six-bit characters. The zone bits of the characters were used to indicate the sign of a number, and the last digit of a number contained its sign, also indicating the end of the number.
This also explains the funny way EBCDIC encodes letters, because it was influenced by those encodings.
Singer made a series of retail systems which were taken over by ICL and rebranded.
The ICL System 10 used 6-bit bytes for decimal and character data.
The ICL System 25 used 8-bit bytes, which were used for full ASCII text, and in arithmetic operations as packed decimal (two 4-bit nibbles holding 0x00 to 0x09).
Singer were fairly adventurous. They made sewing machine in Glasgow (the factory was big enough to have its own Metro station). During WW II, they used their engineering skills to make the Norden bombsight (which was partly electronic), and branched out from there to office systems.
