I recently found in the papers of the Adjutant General what may be a draft version of Special Orders No. 126. This draft is undated and unnumbered, but its text is consistent with what has been speculated to be the likely purpose of the order.

S/O [G is overwritten by S]

The following officers of the Enrolled Missouri Militia having failed to take and subscribe the oath provided in an act to prescribe an oath for certain military officers approved February 15 1864, and to file the same in the office of the Adjutant General of the state on or before the 4th day of the present month. Said act having been duly promulgated in General Orders No 8 current series from these Headquarters, dated March 5th, their commissions are hereby vacated and they therefore cease to be officers of that force from the date afore said.