.45 ACP Case Question WWI?

I just finished cleaning and depriming about 1000 cases. As I was priming the cases, I noticed an odd headstamp. It was F A 6 15. Is this Frankfort Arsenal June 1915?

As an aside, I was shooting a Colt M1903 Hammer manufactured in 1914 today.

Yes. 1st Frankford Arsenal M-1911 headstamp was 9 11, then 11 11 then 12 11 .
1n 1912 there was production from 1 through 12. 1n 1913 only 5 dates, 1, 2, 6, 10 & 11. In 1914 dates are: 1, 2, 4, 8, 10 & 12. In 1915 known production month stamps are: 2, 4, 6. 8, 10, and 12 . In 1916 dates are 4, and 10. The double date continued until 10 17 then the date was just 17
Edited to add the 12 11 date

PetedeCoux,

     Thanks for the reply.  Guess I won't be shooting that one.  Will probably reload it as a 230 gr hardball and leave it on a shelf.  Brass primers are correct?

     I had gotten a bunch of old ammo in a deal (cases, reloads and new).  After I sorted everything out and cleaned it all for reloading, I found this case, guess I will be looking closer at the headstamps.

AJ

AJ: I think you’ll find that early FA case was adapted to the bastard .204 in primer used in that period for the .45 ACP case. Perhaps best to leave it as is as a show and tell. Jack

I had already seat a Winchester Large Pistol Primer in it when I figured that it was different.