RE: New Sig P-51B 90-100 size??
it really doesn't matter if they were not fitted at the factory or not. The whole point is the B/C models got the Malcolm Hood and it a rather good quantity at that. Many 8th and 9th Air Force P-51B/C fighters got the hood. In fact the Malcolm hood was so common on the Mustang III that the Brits actually released a Flight Manual Pilots Notes on it, AP 2025G (& H)-PN from June of 1944
What most people DON'T know is the Malcolm Hood fitted Mustangs are/were considered to be the best dog fighter of all the WW2 Mustangs. Other than the gun jamming issue (caused by the mounting of the twin .50s per wing), the P-51 B/C with the Malcolm hood offered better visibility, lower weight, slightly faster, slightly better handling and suffered from none of the structural problems which impacted the D due to the modified wing design. The P-51 B/C was also more forgiving when flying close to the edge of the flight envelope and was easier to recover when it departed the envelope.
Let us not forget the Brits actually gave North American the idea of fitting the tear drop to the back of the D, when they fitted the canopy from a Hawker Tempest to the 'Stang.