Old-school buttermilk is the milk left after churning butter and is not today's 'cultured buttermilk'.
A recent answer to the question about what to use for 'sweet milk' mentions :
Buttermilk was what was left after the soured milk had been churned and the butter removed. There were always small particles of butter left in it
For years I had assumed that skim milk was the best substitute (as it's milk with the fat removed), but this suggests that it's both soured and has a little bit of fat left (but not even close to homogenized).
Is there something roughly equivalent available today, or something that I can make to approximate old-school buttermilk without churning my own butter**?
** It also hints that today's 'sweet cream' butter is not the same as butter in the old days. I don't know if 'cultured butter' might be closer, or a blend of cultured & sweet cream butters.
Clarification: I am not looking for a replacement for modern 'cultured buttermilk'. I'm quite aware of the substitution for today's buttermilk when baking of using milk plus an acidic liquid, or of thinning yogurt. It's possible that this is also a good substitute for historical buttermilk; if so, please acknowledge in your answer that you're aware that they're different. If you've spent time on a dairy farm, please let us know if the dairy was using fresh or soured milk for their butter (because everything that I've found said that it was made with soured milk historically).
From The Settlement Cookbook (1945), in the discussion of dairy products (pp. 45-56):
Cream is the fat that rises to the top of the milk if left standing. For Whipping Cream, see page 498.
Skim Milk is the milk left over after cream has been skimmed off.
Buttermilk is the liquid left over after cream is churned into butter.
…
Cultured Sour Cream and Buttermilk may be obtained from most milk dealers.
In this case, it doesn't specifically mention that soured milk is used, but I've seen other references from the late 1800s to early 1900s that said that butter was made from soured milk. (and one of them would occasionally say 'new milk' instead of 'milk', suggesting that their use of 'milk' was sour milk or buttermilk. I suspect that before refrigeration, it's quite possible that milk left to separate into cream would sour by the time it was finished. They do mention in that same section:
Sour Milk is valuable in cooking and may be obtained by keeping milk (preferably raw milk) undisturbed in a shallow covered pan at a temperature of 90° to 100°F. until it becomes thick and clabbered. If it sours too slowly it becomes bitter.
Best Answer
Given the variabilities in "buttermilk" from place to place and time to time, you should get sufficiently equivalent results by substituting modern cultured buttermilk. That's the job it was designed to do.
Recipes from the early 19th century and before are notoriously vague. They were generally written more as reminders of something you already knew through experience, rather than detailed instructions for creating it from scratch. Quantities and temperatures were much harder to control, and so recipes basically assumed you'd recognize a dough with enough liquid or a sufficiently-roasted quail. (One of my favorite instructions from an 18th century cookbook: "Cook until it is enough".)
Given those wide margins, you should find that simply using cultured buttermilk will make the recipe work. It's true that it will lack the tiny bits of butter present in "true" buttermilk, but there isn't enough to have a radical effect on the result. The really important parts (dairy protein, water, acidity) are present, in about the same quantities they would be in true buttermilk.
You'd certainly notice the difference if you were to just drink it straight. If that's what you want, you're just going to have to find somebody who makes it, or do it yourself. I've found it's often available at the kinds of farmers markets that insist on local producers. Personally, I find it nasty, but YMMV.
So if you're trying to revive an old recipe, just start with commercial buttermilk. You're going to have to tweak it from there anyway. If it needs richness, add butter, but probably only a teaspoon per cup.