From what I have read, you are correct in did not come in production Camaro. I have seen this block come up for sale a few times. I also see it described in Hooper's 1969 Camaro Reference book (page 291). They also note that the NOS one in the book was about to be installed in a 69Z, and thanks were given to Al Maynard Jr. Maybe he is the prior owner?
The block has been on eBay. I think the last time I saw it for sale it had a 20k asking price.