The latest hardware revision that's expected to be the mainstay for F9 flights going forward. Has better performance and addresses some reusability issues discovered in earlier blocks.

But if they weren't on this flight, how sure is SpaceX that they are more reliable?

But yeah, that's just a question from my mind. This launch was amazing, the future in the making right here.

As far as I know, the center core was a block five one, but they didn't intend to refly it anyway, from what Elon said in the press conference. Maybe they intended to study it? If that's the case, there will probably be another "expandable" block five flight.

According to reddit, even the center core (B1033) was Block 3. No block 5 cores have flown yet.

https://www.reddit.com/r/spacex/wiki/cores#wiki_.5Bfh.5D_b10...

The plain is to lock down block 5's design so it can be man rated which in itself would make it more reliable. This will probably not happen until they fly them a few times though so that wouldn't mean the first block 5s would be more reliable.

I recall reading or hearing yesterday that space-x no longer plans to man-rate any falcon-9s instead reserving that for the BFR.

I believe that was in reference to the Falcon Heavy, not the F9. The F9 is expected to be man-rated for NASA's commercial crew program.

Version 5 of the falcon design.

To be perhaps slightly more accurate, it's like a major version 5.

Almost (if not) all missions include changes of some kind; the manufacturing blocks each have significant, incompatible differences in parts.

Upgrading the grid fins would be a minor version change, modifying the engine to increase its throttle depth a major one. I don't know, but it seems like there would need to be significant re-tooling for each new block, and many parts would be incompatible with previous blocks.