Many automobiles use a body designation like A-Body or B-body which helps to group a larger category of vehicles into a one that share the same basic chassis.
Like, as many reading this probably already know.. the F-body designates the Chevrolet Camaro and Pontiac Firebird. And a J-body was built up to be a Chevy Cavalier, and Pontiac J2000, Sunbird(again), and then Sunfire. All mostly the same car. Generally speaking, they share a floorpan and mechanicals.
So OK then, the H-body was the Vega, Astre / Monza, Sunbird, Skyhawk, Starfire right?
Well, not so fast...
First off, were going to ignore the use of the H-body designation, later in the 1980's for some full size cars.
We'll deal with the more well known and collectible H-body from '71 to '80.
Here's some info:
Sometimes* when GM upgrades a chassis enough, but it's still mostly the original architecture, they'll give the platform a second designation.. For example...
A very recent nameplate, The Chevrolet Cobalt, was built off of the "Delta Platform" Which is sort of a modern way of saying D-Body. But a platform is a little more modular, ie; it can be expanded to more vehicle types than the old 'body' way of making chassis. The Pontiac G5 was also built off of this platform on the same production line. (Lordstown,Ohio). Like the 78-80 "HS-Body".
The Cobalt ran for 6 years or so until production ended and the new and improved Chevy "Cruze" came out.
The Cruze bear's little resemblance to it's predecessor, but underneath it's new skin is the Delta Platform, but with just enough upgrades to get the designation "Delta II".
*The F-body chassis was changed extensively in 1982. Including the torque arm rear axle design from the H body, but it remained designated the F-body, although not much if anything interchanged from the '81 and earlier chassis. So I guess this only happens sometimes, when GM needs it to happen.
Back in 1970.. The chassis created to build the Vega and Astre on, was called the "H-body".
By 1973, GM had a top secret expansion program in operation for the chassis. The would upgrade it to support a newly styled and heavier body. In 2 configuration's, a futuristic styled hatchback, and a 'notchback' coupe body. That would be shared by four makes.
The redesign was to support a whole new line of drivetrains, including a possible 2 rotor Wankel Engine, and/or V6 or even V8 power. The rear axle attaching point's were revised for a whole new "torque arm" setup.
At the same time, it needed to retain the same basic architecture because the Vega and Astre would continue be produced and would be built off this newly upgraded chassis with no changes to the body design of either.
So it was still an H-body but not the old H-body .. So what to call it?
The "HS" was the new designation.
Many publications refered to them as the.. "H-Special" One or 2 magazine articles I have suggest the
"H-Super" designation. As-in these are "Supercoupes". But of course, not to be confused with the Ford Thunderbird, which laid claim to the designation many years later.
Here's an example of the use of the term with the H body..
"Road Test magazine, in its 1976 "Supercoupe Shootout"—Alfa vs. Mazda vs. Lancia vs. Saab vs. Cosworth Vega"
As of this writing, I'm not partial to any particular designation. Personally, I tend to think it was just H special, but if you called them HS Body's that would just simply be easier.
And even easier than that, just call them all H-body's and forget about it!!!!
So... just in case you wondered.. The Monza, Sunbird, Skyhawk, and Starfire were built on the HS-Body chassis.
Just after the first model year of the revised chassis, the Wankel engine idea was canned, and it was possible to lower the height of the transmission tunnel to gain some interior room...
...but that was a minor tweak, and would't require any further chassis designations.