What was the very first device to have an RS-232 serial port
The question is a bit misleading, especially when tying it to 1960.
RS-232 is a standard. Standards are usually made after something has been invented and used first. Their purpose is usually to either unify a varying landscape or turning de facto usage into standard (*1). Think of bolt dimension, tube sizes or screw driver heads.
RS-232 is such a standard codifying and unifying existing industry usage and terminology.
As such there is no real 'first' that can be pointed out without doubt. It's kind like a quantum problem. It's like the cat in the box. prior to opening the box, noone knows if the cat is alive or dead. Or in this case, there can't be no device adhering to RS-232 before it got recommended, but as soon as it was published, a lot of existing devices were compliant and instantly became RS-232 devices, or not ... Getting a common classification is the whole purpose of such a standard
Serial communication using asynchronous is way older. The beginning of what after several steps became RS-232 was Baudot's "Système de télégraphie rapide" which in many ways did forshade upcoming information age - for example by being the first to use a binary representation for encoding and transmission (*2). Like telegraph before it used a current loop for the signals.
Development evolved over the next 100+ years. While Baudot's first machine was semi synchronous (the operator had to type the next character every time the keyboard got release from the previous character, later became truly asynchronous.
While original current was around 60 mA, it got lowered to 20 mA when the 7 bit teletypes got introduced. All still asynchronous over a single line.
With emerging switching and amplifying voltage driven communication became a thing. Various companies developed a great range of interfaces/devices, using different voltages and additional signals.
EIA generated RS232 to unify these into using the same name, workings and meanings.
That article also says the very first devices were "electromechanical typewriters" - which makes sense. The famous ASR-33 was released in 1963, but there were models beforehand. The Bell 103 couldn't have then been the first RS-232 device. So what was the first?
Hard to say, as RS-232 covers a wide range of devices already in existence in 1960. There is no minimum requirement for being either duplex or using any of the control signals. A single pair carrying a word synchronous transmission guarded by a start and a stop bit will be RS232 compliant. So essentially a (Baudot) TTY interface changing current loop to voltage level would fit - and they have been around from the 1870s - for sure including the early days of computing.
*1 - Not to be confused with (interface) specifications made up by companies/inventors/committees to define something new to be implemented later - these may as well become a standard at some point.
*2 - All the way to time multiplexed transmission.