This is a 170Da. It has, as you noted, external trunk access. This was a new feature introduced on the 170Da (and 170Va). The 170D trunk was accessible only thru the back seat. The 170D diesel model was not available pre-war (production May 1949 > May 1950). It is also a 1951 model and the 170Da began production in 1950. The data card and all ID plates should indicate that it is a 170Da.
With regard to the split valve cover, it was furnished on the early 170Da models up until engine number OM636.916-06650. So to answer your question, there was both a split valve cover and single valve cover engine in the 170Da depending upon year of production. This was not uncommon with Mercedes, they made incremental changes as they were available without regard to model designation or year.