Hey I found a reference. Liberation of Terra volume 1, page 115. I did a search for "combine battleships" in the PDF and landed a hit.
It's basically a paragraph talking about the SLDF beginning to experience defections of House troops to the SLDF after the Amaris Coup. Kerensky begins a press campaign for support. Thousands of AWOL House soldiers begin to show up at SLDF bases every week. The relevant sentence is just after that.
Quote: "A few nearly whole companies and battalions managed to defect,
as did numerous DropShips and even a pair of WarShips—both
Combine battleships."
On the next page it says Kerensky targeted some of the Combine's depots, seizing what it needed for war materials. The Coordinator publicly blasted Kerensky for doing this. But a persistent rumor is that it secretly happened with the Coordinator's blessing, and the same might be true of the 2 defecting warships.
So there ya go, and that's a pretty modern book and not one of the old ones.
Field Report 2765 DCMS has a good breakdown of the Combine warship fleet just prior to the Amaris Coup. No battleships are mentioned among the active fleet and it just specifically references Samarkand carriers as the core of each fleet and that the majority of the fleet's hulls are destroyers or corvette classes with a few cruisers. Though within the book you do see a potential clue in a section labeled "Infrastructural Assessment". It says that the Combine has had a limited capacity to build and maintain warships. As a result after the Reunification War it was forced to mothball a number of its warships. But there are signs that the Combine is refurbishing a number of those warships. Due to security at those sites, details aren't known.
So some obscure BB class of Combine design we've never gotten the details on remains a possibility. Though sheer odds of probability at this point tell me we're most likely talking about hulls of Terran Hegemony/Star League origin, possibly dating back to the Reunification War, that the Combine stuffed into mothball reserves.
That makes a lot of sense to me, in part because I think it would be difficult to defect with a fully functioning and crewed warship that is part of the active DCA fleet. A couple of mothball hulls sitting in a repair yard somewhere and a partial crew (and corresponding, lesser security around it compared to an active warship), feels more realistic to me. Even if it happened with the Coordinator's quiet blessing.