Shrouded vs. Unshrouded; P-51B/c Mustangs

My guess, based on conversations with Dad and a few other P-51 pilots,  would be that they were added due to Merlin's tendency to give you a rather exciting exhaust-stack fire due to over-priming...  When you have such an occurance, you have to keep cranking the engine over until it starts.  The shrouds prevented any flames from getting up inside the cowl.  As to when they were added, it looks like they were added in the later B/C blocks (the only difference between the two was where they were built, Dallas or Inglewood)...

That explains the Bs and Cs.  I have also seen photographs Ds and Ks with and without exaust shrouds.  If the above explaiatin covers the entire issue, Iuld I think the question would have been settled by the time those came off the production lines.  Is there a differen for those?