I had this same issue. I bought a new repro m/c for my 69 Bird at D&R Classic, a Camaro resto parts company (they're close to me, I needed it quick, and I could simply walk in and pick it up). They assured me (and I asked 3 times) that the Camaro master cylinder was the same as a Firebird.

That was the last time I ever bought that excuse.

After installing the new m/c, filling it up, then trying to bleed the brakes, I couldn't get anything out of the bleeders. After several attempts, and trying to be as careful as possible with the brake fluid, I finally took the master cylinder off and compared the two. Just like you show above, the Camaro m/c had a much deeper sleeve, so the Firebird booster pushrod wouldn't make contact.

I tried to return it, but they wouldn't take it back because it had fluid run through it. After finding the exact salesman that told me repeatedly that the m/c was the same on a Firebird as it was on a Camaro, I told him they had better stop trying to expand their business by claiming that all their Camaro parts fit Firebirds too.

I ended up going to CarQuest to try and find a sleeve repair kit, and the girl behind the counter asked if I wanted the entire m/c, rebuilt. I told her the issue about the sleeve, and asked her to look to see if the 69 Bird was the same part number as a 69 Camaro in their catalog. She said no, they were different.

Surprise surprise, the rebuilt one was correct, cost $15 (with core), and it's been working flawlessly on my car for 5 years now.