What this 74HC multiplexer does on the board
The SN74HC257N is a quad 2:1 multiplexer from TI's 74HC family — four independent 2-input selector circuits in a single 16-pin DIP package. Each of the four sections selects one of two data inputs and routes it to the output, controlled by a common select line.
Pinout and board fit for the 16-DIP package
Housed in a 16-DIP (0.300" row spacing, 7.62 mm body width), the SN74HC257N is a through-hole part — it plugs into a standard 0.100" pitch socket or solders directly into plated-through holes on a two-layer board. The 16-PDIP supplier package code is the same footprint. Output drive is rated 7.8 mA source and 7.8 mA sink at the supply rails — enough to drive a few HC-family inputs or a low-power LED, but not a bus or a relay coil without a buffer.