Well, "alternative to the flying suit" would be a strong term. It's not as if all flight crew were issued one of each suit, including an HBT one, and they got to choose what to wear - not the case. When one is trying to represent the typical aircrew dress the HBT coveralls would certainly be the exception to the rule rather than an alternative. You see HBTs all the time on ground crew but not on airmen that often. It would be like reenacting a ground crewman and wearing the A-4 or AN-S-31 rather than HBT or wearing an A-2 just because it occasionally happened.
I do agree some reenactors are "experts" too much but considering many are 250lb 55yr old gunners wearing size 48 -50 suits, it is surprising they would argue about such details.
My use of the word "alternative" was not really a "strong term", rather a statement of fact that is very easily confirmed with a little bit of research, but generally overlooked by the casual observer. Bearing in mind that the men we are talking about were NCOs in the U.S. Army, an HBT suit is not something that would have been in short supply and was not limited in issue only to mechanics or tank crews. The OP was not trying to represent "the typical aircrew dress" for reenacting, he was trying to identify what a specific individual in his original vintage photograph was wearing. No question the A-4 and AN-S-31 family of suits were widely issued and used by bomber crews, but I would disagree that the aircrewmen did not "get to choose what to wear". Other than the mandatory survival gear, they certainly did. Photos of bomber crews in theater, either before, or after, a mission provide ample proof of the many combinations of flight clothing that were employed. Below is a B-24 crew from the 458th Group, 753rd Squadron in mid-to-late 1944. Of the ten men shown, their outer layer of clothing includes at least six F2 electric suits, one B-10 jacket and two HBT suits (note the notched lapels and front hip pockets with flaps on the men standing at far left and far right). I think if you look for the distinctive characteristics of the HBT suit, compared to the conventional flight suits, when viewing bomber crew photos, you'll be surprised at how many you will see in use.
As an aside, in the PTO, HBT suits were widely issued to Navy and Marine aviators as a true "alternative" flight suit, but that's a subject for another thread and another day.
By the way, thanks for that great photo 38Driver!