This is a Class A Euphonium with 3 valves. Serial number is 12xxx.
This instrument belonged to a music teacher in the UK for many years, and is in surprisingly good shape. The 3rd valve does not appear to be original, and has a longer stem and a different valve cap. The case is in excellent shape, although it is a very snug fit and it is possible this is not original.
The shape is in the "squat" type design typical of this era of Salvation Army instruments and it is a lot of fun to play. Interestingly, it is more airy after a few cleans, which can likely be attributed to removal of material that was blocking air leaks and holes.
Record indicate this instrument was manufactured in 1912.
This Euphonium is currently undergoing restoration, as can be seen in one of the images.