Don, I have two of these Desoto Hemi's. The one I am installing in my 49 F-1 its a 330 inch motor and the other is a 291 out of a 55 Desoto. In all of the research I have done I have always found that information to say that "ALL" Desotos came stock with the spark plug covers. Interesting enough that you have the second one with no provisions for those covers that I have seen. The valve covers on my 291 also had no lugs to recieve the covers, this motor I took out of the car myself and I am as sure as anyone can be that the motor was just as it came from the factory. The car had plates on it from 1961 and the odometer showed 36,021 miles. I dont usually hold a lot of stock into those things but it seems reasonable that a car built in 1955 and only driven for 6 years could have that small amount of milege. The valve covers themselves looked like they had never been off of the car, but who knows?
Another interesting thing about those covers is the fact that they had dimples like the Chyrsler engines used in "Industrial" applications. These dimples were to clear adjustable rockers, that were only available on Industrial and Marine engines. No place in my research shows any industrial or Marine Desoto engines.
These covers were in such good shape and also a little unusal so I am going to run them on the 330 inch motor, heres a pic.....
Roland