Gil -
Sometimes mountain howitzers WERE what was meant, though these small English rifles were, indeed, sometimes called mtn. howitzers in error. The report for Wachsman's / Thurber's battery at the end of August 1863 states:

"Two sections of this Battery are absent on detach'd serv to the field[?]. Lt. A. Wachsman with 2 2 9. Engl Rifle Guns and 21 men in the Border Dist from June 16. Lt. B. F. Newgent with one 12 pdr Mount How [,] one 2 " 9 Engl Rifle Gun and 18 men in the Centl Dist from July 20" [etc]"

That's the most detailed inventory I've found so far: 3 rifles and 1 mountain howitzer, in 2 of the 3 sections. We must infer that the third section (under Thurber) had a 4th "Engl Rifle Gun" and a second "12 pdr Mount How".

With Br.Gen Thomas Ewing in operations against Jo. Shelby in October, 1863. By October 21 (at Diamond Springs, Missouri) Ewing stated he had with him: "two small rifled steel guns and two mountain howitzers" referring to Lieut. Wachsman's section of Thurber's Battery, and these howitzers may or may not have also been part of the militia battery.

Finally, from Warrensburg, on 26 July, 1864, Maj.Gen. Alfred H. Pleasonton: "There is a battery here of four 3-inch guns and two mountain howitzers. The latter are of no use, and the whole battery is of little moment for this service." This is Battery L 2d MO, and the General truely means mountain howitzers (not a mistake for the English rifles). And to my disappointment, the four 3-inch rifles are almost certainly wrought-iron Ordnance guns. The English guns are "gone".

Woodruff guns were indeed used in Missouri, almost a lot, but never in Wachsman's / Thurber's battery.