Enterprise opening credits question

What’s the ship that appears very briefly between the Phoenix and the Enterprise?

It looks like the aft engines of a (detailed) TOS era Romulan Bird of Prey to me, personally, but I doubt it’s any sort of previously seen ship and was probably just meant to show the evolution of Earth ships from the Phoenix to the Enterprise.

I agree. A transitional type of ship, I guess, maybe the first widely-produced warp-capable starship (according to TAS, the Bonaventure, but that’s not canon, and that ship looked different anyway).

Probably the one Archer’s father worked on.