I guess I went down the wrong rabbit hole! I had missed the fact that it registers to 400.

Taylor of Rochester was founded in 1851...so this instrument could very well be from the Civil War.

I don't know much about steam, and the pressures that boilers saw, but without fittings I don't see how it could be a pressure gauge for a boiler. It would have to be measuring temperature or ambient pressure (which wouldn't reach "400" if measured in inches...would it in metric units?).

Although there is a company named "Accuratus", I don't find any type of instrument called that. Accuratus is a materials company (ceramics). I think that it is interesting that the piece has the words on it that are the names of three. I wonder if it may have had parts from all three sources.