The only thing Mild traditionally meant was that it wasn't aged. 'young' = 'mild'. Had nothing to do with strength, roast, hops or anything else. Mild's were commonly +6%. Sarah Hughes is actually an outlier b/c of the massive amount of crystal malt (25%) that goes in and the lack of any brewing sugars. Historical milds never had any where near that much crystal...if any at all.
As for stout, roasted barley wasn't used widespread until the 20th century and then not really until WWII. The only thing that stout meant was that the beer was a porter that was STOUTER at that individual brewery. Meaning Truman's stout might not be stronger than Barclay Perkins porter. Its relative. People seem to fumble over roasted barley still. There are many many stouts made today that have no roasted barley and are thoroughly stout's.