“The Horizon was one of the most important ships and classes in spaceflight history. The flagship for early Earth space exploration, the UES Horizon journeyed to the edge of the galaxy, encountering many new worlds in its numerous voyages. At the end of its commissioned life it was retired to Star Fleet’s spaceflight museum on Memory Alpha.”

“.. the Pollux and its fellow Messier class ships were ..refitted for military use. The Pollux saw considerable action during the Romulan War.”

The Messier class cruisers were built in two distinct different variations, when  the Messier specification was issued two different shipyards submitted design and built bids. To satisfy competing political interests the order was split between the two yards (one terran and one martian). Both yards used identical equipment, and broadly similar dimensions, but the execution of the design was considerably different. Although the approach met political needs and also served to drive down costs to a degree by encouraging competition, it did lead to a duplication of parts and components which led to overall increased costs.