It was used exclusively for a few months, until such time as the SSAN replaced the RA/US/NG/AUS numbers.
I went RA on June 30, 1969 and was given an RA number.
Next day, July 1, 1969, DOD went to the SSN as you said.
Funny thing, as much as they drilled into us, "always remember your service number!", I still remember my RA number 40+ years later :-)
If I may add an anecdote, re: remembering your service number:
Our Drill Sergeant (one SSG Turnbull, which, of course became Turdbull, behind his back, mind you) required each of us, at meal time, to jump flatfooted onto the small open vestibule-type entrance of the messhall, scream at the top of one's lungs: "Drill Sergeant Turnbull, Private Doe, RA********, reporting for my meal!"
If one stumbled or could not immediately remember one's SN, one was subject to dire consequences, usually a thousand pushups, before one could rejoin the mess hall line.
God help you, if you announced US or NG, whether you stumbled over it or not, you were subjected to not only thousands of pushups, but had to go throught the gorrilla bars and the sawdust pit at least twice, before admittance to the mess hall.
The US indicated a draftee and the NG indicated National Guard.
The RA was for Regular Army, indicative of a voluntary enlistee.
It took me about three times to get the routine down perfectly, but of course, the DS, being the buttheads that DS's are (I later became one) simply found another query to trip me up on.
Oh, for the good ol' days!
Army serial numbers; still remember mine from 60 years ago. RA all they way...
Army serial numbers; still remember mine from 60 years ago. RA all they way... Oh about question . Laundry mark; last name first letter last 4 digits army serial number