VERY FINE. FEWER THAN FIVE TRANS-RIO GRANDE COVERS ARE RECORDED WITH BOTH CONFEDERATE AND MEXICAN POSTAL MARKINGS.
Single envelopes (or outer address sheets) were rarely used for the entire trip with the intermediary's address and final address together. F. Groos & Co. was a large cotton broker using the Eagle Pass-Piedras Negras crossing. In September 1863, some letters addressed to the care of his firm at Piedras Negras were posted at the Monterrey post office in Mexico, then forwarded across the river with C.S.A. postage added to the letter. Fewer than five such letters are known with both Mexican and Confederate frankings and postal markings.
Illustrated in Special Routes (p. 158). Ex Birkinbine