20,302 Z/28's to be exact. Of a total 243,085 69's produced, that's 12% of them being Z/28's. Not "rare" by some other option standards, but still a very desirable option.
There were a couple of Z/28 trim tag codes; X33 - style trim group with Z/28 package, and X77 - base model with Z/28 package.
Your 69 is still worth more than a plain jane model, bot only a fraction of what it would have been worth with the original engine.

 

I have probably read 10,000 of your 16,000+ posts and I don't think I have ever seen a mistake before. Assuming the below numbers are right. 8.5% is a little closer. I suck at typing but I can spot a math mistake in a second.

 

Oh man, I was accidentally dividing small number into big...long weekend I guess. Oops, it's 8.35%, you finally got me.