Note: Red Feather butter, according to its label, has 100 mg of sodium per 14 g of butter, which would be 0.25 g NaCl/14 g, a 10.5% salt solution by weight in a butter with 80% fat and 17% water (1.4 t salt/lb of butter, 1.76% salt by total weight of butter). That gives an active water of a shade under the 0.94 of a 10% salt solution (approximately 0.9366), and 0.94 is the minimum amount of active water needed for the least sensitive strains of botulism to grow. The high-heat processing of the cream is sufficient to take care of the other potential food poisoning organisms that can grow at lower active water contents, but are more heat sensitive (listeria, staph, molds, etc.). However, the guaranteed safety from botulism is apparently based on having a precisely formulated butter with an absolutely known and uniform water and salt content in every can.
This is why everyone who sells canned butter offers only salted butter in those cans--combining salt to reduce the active water with UHT processing of the cream and a sterile manufacturing line takes care of all potential food poisoning organisms. Without the salt, it isn't possible to assure safety from botulism and some other less heat-sensitive organisms, and they would have to apply too much heat over too much time (too high a total thermal load) to be able to can good-quality butter. A small error in the total water content of your butter/weight of salt per pound or failure to obtain uniform mixing of your salt, though, and botulism can grow. Failure to obtain a sufficient total thermal load with complete and uniform heat penetration of the fat mass, and other organisms can grow. This is why the USDA is never going to issue tested home-canning instructions for butter. If you want to get yourself some basic lab test equipment, a precision scale, a reference manual on both max temp and minimum active water that various spoilage organisms can survive, and work out how to apply a uniform thermal load of the requisite amount to your butter and maintain sterility in those jars, I'm sure you can devise a safe canning procedure for your butter for yourself that will at least give you something in the ballpark of commercial results, but I'm going to let the commercial canneries do it for this one because there's not enough savings to be had home-canning grassfed butter to justify all the work, at least not for me.
If you want to live dangerously and thumb your nose at both the USDA and the commerciall canners, then at least do not home can butter with a salt content of less than 2 teaspoons (11.4 g) per pound, 2.5% by weight (which will still give a 12.5% salt content in your water content/active water of around 0.925 even if the fat content of the unsalted supermarket butter you start with is below the minimum standard by a couple of percentage points at 78%, giving you a 1.5% margin of safety against botulism in your active water), and mix the salt thoroughly into melted butter. Canning unsalted butter is an invitation to disaster. There is not a single commercial brand of canned butter that is unsalted, and there never will be one. (I know; I scoured the world for one before I finally worked out the sciene of the thing.
) Canning commercial salted butter may or may not be safe since the added salt varies from 1% to 2% and the water content from 15% to 18% with no way for you to know exactly what either is, so that's basically just a different invitation to disaster. Canning homemade salted butter that hasn't had all the trapped buttermilk thoroughly
worked out of it with Scotch hands/butter paddles and the salt thoroughly blended into it while melted to assure even distribution is also an invitation to disaster. Homemade butter can easily still have a 25% or higher water content. I would only try to can that if it was both made as a cultured butter and I added another teaspoon of salt per pound or I had enough of a basic lab to test the water content of the batch before canning. Yeah, your butter would be very salty, and you'd want to knead it around in cold water to reduce the salt before using it, but it would at least not have a risk of botulism. Commercial canneries have the technology and the testing to be able to add the absolute minimum necessary salt to their canned butter and still produce a guaranteed safe product; you don't (or at least the vast majority of people don't, and any who do wouldn't be here trying to find out how to do this).
(BTW, any of you ever run into a youboob butter-canning "expert" who actually knows/mentions that you can only safely can well-salted butter or clarified butter/ghee following their directions? I haven't, which is one of many reasons I don't go to youtube for canning information. There is nothing more dangerous than just a little knowledge, the kind of half-information that causes people to assume that anything processed as long as canned beef is sure to be safe, or to fail to see any reason why just baking jars of butter in a 300ºF oven for half an hour wouldn't sterilize the butter when, after all, a pressure canner only goes to 250.)
"A little learning is a dangerous thing;
drink deep, or taste not the Pierian spring:
there shallow draughts intoxicate the brain,
and drinking largely sobers us again."
If you want to go beyond the canning recipes/rules on the USDA site and in the Ball Blue Book, then you need to learn a lot more than just what is in those. You need to understand the basic principles and science behind their directions and recipes. You need to do a lot of learning and do thorough research in the scientific literature/food industry journals on whatever else it is you want to try canning. If you want to go really far beyond them, you will need an accurate pH meter and an active water meter, both designed for use with food, as well as microbiologic controls and the means to get culture results with them.