My take on this issue is strongly influenced by the well-respected aviation author William (Bill) Green's book 'Famous Fighters of the Second World War' in which the following paragraphs appear on the Mustang's development:
"After conducting extremely successful trials with the two XP-51s the USAAF began to evince a little interest in the North American fighter, ordering 150 P-51s under the name
Apache, armed with four 20mm cannon and equivalent to the RAF Mustang 1A."
and
"The time was then November 1942, and it was to be a year before the first group of P-51B
Mustangs (this name having also been adopted by the USAAF in preference to the
Apache) were to be available for combat. In the meantime a new version of the P-51A had been evolved specifically for dive bombing in the spring of 1942, fitted with wing mounted airbrakes, a 1,325hp Allison V-1710-87 engine and designated A-36A............The dive bombing equipment of the A-36A increased gross weight to 10,700lbs and reduced maximum speed to 356 mph, and the type was only moderately successful. The dive brakes proved unsuccessful and were eventually wired shut......One A-36A (EW998)* was supplied to the RAF in March 1943 for experimental purposes."
Over the the years, extensive other readings have led me to understand that, whereas the P-51s (ie fighter role) were called Mustangs, the A-36A (ie close support role) retained the the original USAAF name of Apache, as a means of differentiating their different roles, which seems a sensible move and would probably have benefits in the logistical supply chain.
*Presumably the one in the picture above, clearly showing the dive brakes deployed.