I'm not an expert on this by any means but two different parts of my background might come to play here.
First, as the Air Corps guy...the A-2 Jacket originally dates to about 1935 +/- (I have the exact date somewhere). It, in turn, was a replacement for the Type A-1 leather jacket, also made of brown dyed horsehide and first issued about 1926+/-. (Note that, in the thirties, the Bureau of Aero, USN had a similar leather flight jacket to the A-1 but in green/OD dyed leather-don't confuse this with the Air Corps Type A-1 wool liner, in green/OD, that could be worn under the A-1 leather outer jacket) The A-1 jacket has similar lines and cut to the A-2 but has a button up front and a knit collar, like a letterman's jacket. The knit collar is the same brown wool knit as the waist band. The invention or, more correctly, the ability to produce them in quantity, made the "interlocking fastener" (zippers to those who don't know) an easier way of getting jackets on, off or slightly open, while wearing flying gloves, than the button type A-1 which is, perhaps one of the basic reasons for the change to the A-2. The Air Corps was so small in those days that the supplier(s) had no problem producing them in horsehide as the specs called for and, this was still the case in the mid to late 1930's when the A-2 replaced it as STANDARD. Though OBSOLETE, some A-1's were still in the supply chain when the War started. So, if you see a leather flight jacket in a photo, that appears to have a button-up front and no leather collar, it is probably an A-1 or, as often the case, a privately purchased one available from many of the period Officer Outfitters. But also, keep in mind, that many A-2's (as well as early shearling B-3's) had been made and issued prior to the outbreak of WWII so seeing them in photos that seem out of context of their commonly accepted dating of WWII, is not unknown. The early WWII era Navy leather flight jacket, similar to the A-2 and the predecessor to the Navy G-1, was, if memory serves, the M-442, which differed from the A-2 mainly in its brown mouton fur collar. The original specs here too, called for horsehide. However, those I've seen over the years, seem to be dyed a darker brown than the Air Corps A-1 or A-2's. As the number of horsehides was no where near the requirements of jackets needed, by as early as late 1941, specs were changed to cowhide. The Office of Strategic Materials started a huge program with ranchers to provide appropriate numbers of hides for military usage which, I imagine, included leather for Army boots and such as well. (As a note of interest, the Eastman Leather Company jackets, though not the cheapest, are probably the best repros available and, can be ordered in several of the original leathers including horsehide or even sealhide as some test A-2's were made in. The English chap behind the company has spent years collecting original spec info on virtually all USAAC, AAF, USN, RAF and GAF leather jackets, gloves, hats etc. I'll contact him and see if he has any material in his archives defining leather colors for the US.) When the Government and QMC development offices switched, in 1943, to the new, layered principle, of cloth clothing and eliminated leather flight clothing, ranchers/tannerys who had set up these huge facilitys were stuck holding the bag, with millions of useless hides. Legal action over these cancelled contracts lasted well beyond the end of WWII.
My other angle to this is that my web business, Wakeda Trading Post (http://www.wakeda.com
), sells tanned hides and rawhides to Native Americans and crafters. The modern tanning process, sometimes referred to as Chrome Tanning, does turn out a leather that is almost a pale blue/gray. Bleaching turns the hides to white and they are then dyed, in vats to produce the required colors, or occasionally, smoked for the beige colors. On the other hand, in the original Brain-Tanning process, most well known thru Native American tanners, the hide comes out a white color. Most hides are then smoked, often repeatedly, to soften them. The reason they are smoked is that, if a non-smoked, brain tan hide gets wet it becomes permanently stiff. If they are smoked a number of times (each smoking adds a little yellow to the hide) then the hide can get wet and not stiffen up.
Finally, an aside on MacArthur...if I remember correctly, he had resigned his commisson in the Army, at some point between Wars, to become the head of the Phillipine Military though I don't recall his exact title. This is similar to Chennault's position with the Nationalist Chinese government-he too resigned his Commission to take that position. I believe that is where he first wore a "scrambled egg" hat as part of that country's dress uniforming. As to the leather flight jacket, he was known to frequent the Officers Club at Clark Field during his long period of service both to and in the Phillipines and held life-long friendships with many in the early Air Corps, probably the source of his A-2. Almost all of the well known names in the Army Air Forces of WWII had rotated thru Clark in the twenties and thirties as junior officers, when it often took 10-15 years to advance a single grade!
Hope this isn't too much so please pardon my verbosity.
the Air Corps guy