I am quite familiar with Wachsman's/Thurber's Battery Missouri State Militia Light Artillery and the battery it would subsequently form the core of - Co. L, 2nd Missouri Light Artillery - as I plan on publishing a history of the units in the next couple of years. (My great-grandfather served in both units.)
It appears only the quarterly ordnance reports for June/July 1865 will identify what type of guns and which of the batteries manned the guns arming Cole's column as I have never found mention of it in a primary source. Keep me posted!
I have copies of Junius McMurray's military record (as well as all of the members of the both batteries). He was on the staff of Col. Nelson Cole and never exercised actual command of Company L during the Powder River Expedition.