The next Protocyberpunk story I wanted to recommend is a bit more of a stretch than The Space Merchants but I'm prepared to argue for it. It's a short story called The Velvet Glove written by Harry Harrison (the prolific author behind the Stainless Steel Rat, Deathworld, Make Room Make Room, Bill the Galactic Hero and like a hundred other stores) in 1956.
It's short, it's available for free on Project Gutenberg. I think I'm going to spoiler-tag the rest of my case for this being protocyberpunk because it's a fun little piece and even the premise is a little bit of a spoiler that wouldn't ruin it for you but might change the reading experience.
spoiler
The story focuses on Jon Venex, a robot and second class citizen in New York, who gets dragged into a criminal enterprise and escapes using his wits and by exploiting some of the features of his mechanical body.
It has a few of the common cyberpunk elements - the technology, and the way it's fallen into the hands of common people is a big one. 'The street finds it's own use for things' I think applies both to the robots themselves, who now own their own bodies along with the responsibility for their maintenance, and to the criminals who have found a way to exploit the robots' hard-coded drive to protect humans so they can use them in their heist.
There's a class divide, both by wealth and between humans and robots that leaves an underclass of people like the protagonist. This divide is pretty much the primary element of the setting, the bigotry against machines is a major factor in the plot, setting up the characters' circumstances and vulnerability, and also paving the way for Jon to escape when the criminals underestimate him. The scene where the black man saves him from a quickly-forming mob might read as a bit trite now but it was written in 1956. The civil rights movement was very much underway, sundown towns still existed. Emmett Till had been lynched only a year before and similar murders would continue for decades. I've seen it argued that in the early days, the 'punk' in cyberpunk referred more to the authors and their rejection of mainstream trends in science fiction than to their characters, who tended towards being more common criminals than revolutionaries. I figure writing scenes like this one at that time would qualify, though I'm not sure what Harrison would have thought of the title.
In the end, I think it's the way Jon exploits both the criminals' low expectations of him and the technology of his own body to escape and call for help that pushes it towards feeling proto-cyberpunk to me.
Beyond that, I love the little details of the setting - the robot family names being the model or class of robot, the decrepetness of the hotel, even the detail of the power line executions, Alec Digger hiding a diamond, stolen from the mining company, inside his eye. I have a special fondness for the rusted-out robot built of cheap parts by a cheaper company. There's even a hint of the kind of corporate espionage and sabotage we expect in modern cyberpunk, which they use to trick Jon into taking an off-the-books job.