Not quite sure what you mean by"The Heavy Duty Service Replacement Rear Disc Brake kit" or "differrent bolt pattern".
I own a small axle bearing JL-8 rear that has no stamp codes. That tells me it was a bare housing that was sold over the counter.
I also own a large axle JL-8 rear that has stamp codes. That tells me it was sold assembled over the counter--probably dragged around the counter.
When you refer to the "different bolt pattern" I'm guessing you mean the four holes on the flange that mount the caliper bracket or backing plate? This has NOTHING to do with the bearing size, as your pictures only show a flange and not the bearing area. ALL Camaro rear disc housings will utilize the larger pattern--same as the Impala you mentioned. This has NOTHING to do with the bearing size but rather the proper bolt pattern for the Corvette rear caliper bracket. Both small and large bearing axles used the big pattern because of the need to match the Corvette pieces.