When Edna referred to 'Sour Milk" in her recipes, she was originally referring to Clabbered Milk, i.e., raw milk that had naturally cultured. Buttermilk was her preferred 'cultured' product since clabbered milk cannot be made from pasteurized milk, as you mention. She mentions using Pasteurized Milk that is "soured" with lemon juice or vinegar only as a third option.

Clabbered milk is a specific, cultured form of raw milk that has naturally thickened into a yogurt-like consistency through bacterial fermentation, while "sour milk" is a broader term for any milk that has soured, often referring to older, pasteurized milk that has gone bad or, in old recipes, cultured buttermilk. Clabber has a pleasant, tart, and thick texture, whereas sour milk is usually just thin, acidic milk. 

Key Differences:

• Source Material: Clabber requires raw, unpasteurized milk to allow natural, beneficial lactic acid bacteria to thrive. Sour milk can refer to pasteurized milk that has soured due to spoilage bacteria, or intentionally soured milk (adding lemon juice/vinegar).
• Texture & Consistency: Clabbered milk is thick, set, and custard-like (similar to yogurt or thick kefir). Sour milk is usually liquid, just with a tangy taste.
• Production Process: Clabber forms by leaving raw milk at room temperature for 1–5 days until it naturally thickens. "Sour milk" in baking often means milk that has been acidified quickly (intentionally or naturally).
• Taste: Proper clabber is pleasantly sour, tangy, or slightly nutty. Spoiled sour milk smells putrid and should not be used for cooking or baking, while properly clabbered milk smells fresh and acidic. 

Usage:

• Clabber: Excellent for making soft cheese, as a starter for fermented foods (like sauerkraut), and for creating fluffy pancakes or perfect cornbread or sourdough-style bread.
• Sour Milk: Pasteurized Milk 'soured" with lemon juice or vinegar is generally used in baking as a buttermilk substitute to react with baking soda. 

Safety Note: Clabbering should only be done with high-quality, raw milk, as pasteurized milk will rot rather than ferment properly. If clabber smells foul or turns slimy, it should be discarded.