4YA 1923 Atlanta, Georgia

Signal Corps Unit R.O.T.C. Georgia School of Technology

Operator: 4YA, E.B. Rumble

Since 4YA was a signal corps unit of  R.O.T.C., those call signs (4YA, 4KM, 4YD, WX5, CD7) are probably Army call signs of an army net.
See AS2 QSL!

QSL from the estate of 9DCR
K8CX Collection
Photo courtesy of W4ATL
Info courtesy of Patrick Rigg