I have been unable to locate him in another command, nor who Lt. Col. Weems was.

Cash enlisted October 7, 1862 at Calhoun, Georgia by Lt. Col. Weems for 3 years or the war, assigned to the 1st Engineers December 15, 1863 by S. O. 297, A. & I. G. O.

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Some additional information on Col. Talbot, commanding the 1st Engineers.

The 1st Regiment Engineer Troops, C. S. A., completed its organization April 1, 1864, after having served for a few months under the command of Lieut. Col. Talcott. It was comprised of ten companies, A to K, formed under an act of Congress approved March 20, 1864, which were composed from other organizations and of men assigned from various States by Special Orders of the A. and I. G. O.

Regiment organized April 1, 1864 as a complete organization. Previously served under Lt. Col Talcott for a few months while in process of formation.

Thos. M. Randolph Talcott, Lt. Col. & Col., age 48
W. W. Blackfor, Maj./Lt. Col.
Payton Randolph, Maj.

Lt. Col T. M. R. Talcott ordered to report to Col. Gilmer, Engineer, July 25, 1863, to take rank July 25, delivered by Gen. Lee, for duty with General Lee

Date of appointment as Colonel to 1st Engineers April 1, 1864 near Orange Court House, Va. Engineer by profession, served as Captain of Engineers during the first year of the war & afterwards as a Major & A. D. C. to General R. E. Lee during the campaigns of 1862 & 1863. Is now commanding the 1st Engineer Regiment - Promotion recommended by General R. E. Lee

Paroled at Appomattox Court House, Virginia April 9, 1865

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1st Engineers Regiment was organized at Richmond, Virginia, during the fall of 1863 with men from North Carolina and Virginia. In December about 250 of these men were repairing roads in the Richmond area. Later the unit served as prison guards, then repaired railroads and bridges. During the winter of 1864 - 1865 it built Bevill’s Bridge over the Appomattox River about twenty-five miles above Petersburg and sent a pontoon bridge to the Richmond and Danville Railroad Crossing of the Staunton River. In preparation for the spring campaign, it readied another pontoon bridge for use at the site of the roads from Richmond and Petersburg to the several crossings ot the Appomattox. The unit last 2 men killed and 28 wounded during the war, and when the Army of Northern Virginia surrendered 233 were present. Its commanders were Colonel Thomas M. R. Talcott, Lieutenant Colonel William W. Blackford, and Major Peyton Randolph.
http://www.angelfire.com/rebellion2/engineers/history.html

See also: http://xroads.virginia.edu/%7EUG97/flags/engine.html