Hawkins Cheezies? 
Like the donut company mentioned above I wonder how many companies exist with just one sku.
Carmex lip balm used to be just one sku but they have since made available various packaging types and scents.
Lake Country Candies on the other hand makes only one product avaialble in only one size. The 8oz bag of candy raisins.
But they have large and small sizes of plumbers. 
There is a restaurant in Athens whose only product is pork kebabs.
I am tempted to say Saudi Aramco.
However they sell both oil and gas, and are currently beginning a diversification to try to get way from the curse of oil.
But when you are buying oil there are not many choices, and what you get is determined by where it came from. Oil from Saudi Aramco is a single commodity product. You just say how much you want. As far as financial success, they have more money than God.
If we went down the line of major oil production companies, we would soon find a pure play oil producer, and they would win any money earning prize against all comers. The well known brand names may be bigger, and have a huge range of products, but there is a huge set of production companies that do nothing but produce.
Even Huy Fong makes different sauces:
- Your classic sriracha
- Chili garlic - a chunkier, more garlicky hot sauce
- Sambal oelek - chili paste
I’d like a plumber with no buttock cleavage, please.
Guinness?
Ooh, I think I’ve got one. Since 1932, the Tower Optical Company has been making those wonderful coin-operated binoculars. They have recently introduced a different base for wheelchair users, but all they make and sell is one style of coin operated binoculars, either on the traditional or ADA base.
Wouldn’t count per the OP because they have a couple of different styles of brew now.
This thread reminds me that there are stores that sell nothing but harps. No other instruments, just harps. And they have a lot of them on display and they can be really expensive. Well in excess of 10 grand.
Perhaps I’m ignorant, but are there really that many people buying harps (expensive ones at that) that these stores can surviving selling nothing but? I just think it’s really weird. If they sold other instruments as well I wouldn’t think much of it. But they sell harps and only harps.
Spicy sauce, or regular? With or without cheese? Probably too many variants to qualify for the OP. Plus drinks, and probably sides like chips?
The Victor company in Lititz, PA, has remained in business for 120 years, making nothing but mousetraps. Apparently, nobody’s ever built a better one.
Even ignoring the ‘many types of tyres’, they’re not a single product company, with the travel and restaurant guides as well as the tyres. They possibly win some kind of ‘two least connected product lines’ challenge instead.
No they haven’t. No offense to you, though.
I’m sticking with some kid with a lemonade stand selling nothing but his own, homemade concoction and nothing else. I can’t think of anything beyond it that would satisfy the OP’s insane specs.
…and merchandise.
Yep, I think they should probably get the World Record for Biggest company that makes only one thing.
Guinness makes more than one beer, so not by this thread’s standard.
Even their most well-known brew, Guinness Draught, comes in kegs, widget cans, and bottles and is probably disqualified.
Left and right handed, too.
Those have been on back-order for decades.
And kitchen knives.
There are plenty of distillers who only make one kind of brandy or whisky or other liquor, in one size. Some of those are very successful.
A note on De Beers (assuming “diamonds” count as one product):
Starting in September they will be selling synthetic diamonds. (The kind grown in a lab.) So they are presumably already mass producing them.
For years De Beers has been pointing out that synthetic diamonds are not the same/as good as natural diamonds.
De Beers also has diamond related businesses such as their grading business.