Wheaty,
If you are gaging the thread at 28 tpi, it's not 4BA. If you are getting a 55 degree thread, it's definately not a BA thread form.
The BA series are a bastard thread standard that were effectively a metric based design, made on inch equipment. The BA thread form is 47 1/2 degrees, and it has a (relatively) huge radius at both the root and crest of the thread form, and is a PITA to cut because of that.
Nowadays they are used on a lot of UK electrical equipment, like terminal strips, switch plates, and the like, as well as still being pretty popular with the Model Engineering guys, making little steam engines, which is where I see the stuff pretty often.
I'd bet on the parts having been made with a screwplate, rather than with modern production methods, if it is a British 1850's product. Machine made parts were the exception, rather than the rule at that time.
Chances are beyond poor that there are any standard replacement parts available. Chances are also poor that the nipples were made to any particular standard, as we would know the term, at all.
That pretty much leaves you with the options of modifying the gun to take available nipples, or modifying available nipples to fit the thread on the gun.
Modding the gun is going to be a lot easier, but modding the parts preserves the guns originality. Your call.
Cheers
Trev