.30-06 M1 Armour Piercing

This .30-06 M1 Armour Piercing cartridge was marked ‘experimental thick walled case’.
It’s headstamp is F A . 3 9.
HWS vol.1 mentions a series of experimental armour piercing rounds during this period but doesn’t seem to identify this particular cartridge. Can anybody help?

Hope this helps;

CHris Punnett’s book says under the heading Experimental - Cases: Brass Cases, “A further change in the case manufacturing process occurred in 1939 which resulted in a case with slightly thicker walls. Cases made in 1939 with the new process have a dot in the headstamp at 9 o’clock.”

Just the help I wanted! Thanks Simon.