IBM 1401
The IBM 1401 was a small mainframe introduced by IBM in 1959. It was internally a digit/character at a time machine (a serial computer, in a way), using decimal addressing; it supported from 1,400 up to 16,000 locations of main memory. (They held 8-bit bytes - although that term did not exist then - holding a six-bit character - similar to SIXBIT, a parity bit, and a 'wordmark' bit, used to mark the end of variable sized operands and instructions.)
Instructions contained up to 4 fields: an operation code (one byte - the only mandatory field), an A operand address (three bytes), a B operand address (also three bytes), and a 'digit modifier' (one byte). Three bytes (locations) sufficed to hold any address, as characters were used to hold the first digit for addresses above '999'. Most instructions were memory to memory, which worked well with the 1401's variable-length operands.
It was intended primarily for the processing of data stored on punched cards (the primary medium of data storage in the business world at the time). It rapidly became the first computer used by many organizations around the world, and was the most common computer during most of the 1960s (in installations, not sales - IBM still mostly rented its products at that point). In the middle of the 1960s, half of all computers in the world were 1401s.
It was one of the first IBM computers to be built with transistors, although its clock speed was a relatively sedate 11.5 µseconds; it also used core memory. Its 1403 chain line printer was outstanding; it was still a standard output peripheral over a decade later.
