Daily reminder that the historical research you do for creative writing is more of a guidance rather than a set of rules. If it's not conventional for that era but is still plausible, especially considering character, then who's to say you can't put it in?
Like yes, Karl having shoulder-length curly hair for a man is considerably out of place for the 1950s. However, that's the point of it because Karl is an outsider in the 1950s considering that mental disorders like his (keep in mind neurodivergent is more of an accurate term nowadays) would have been frowned upon by the "perfect" 1950s society. It's also plausible that he can achieve curly hair because hair rollers were, obviously, purchasable for home use and how-to magazines existed (though Karl wouldn't have followed it perfectly).
If it's something like tomato or potato farms in Medieval England, then that's fully inaccurate as tomatoes and potatoes (along with other foods) were only available in the Americas and weren't brought over until after the Columbian exchange. This is also potentially harmful misinformation as it takes away the credit of the native Americans originally cultivating the food by saying "it was always in Europe".
Basically, if it's plausible for the character/setting, considering multiple factors, but not conventional in that era, then you can put it into your fictional story as long as can both justify it and show how it is out of place for that era. However,