Unmarked US 40x311R Bofors

Today I got a US WW2 era 40x311R Bofors case with no markings except for the primer, which is stamped “AOP 21 1943 M38B2” is it usual for US 40mm Bofors cases of this era to have no headstamp? If there was any printed information on the case it is long gone. Thanks in advance for any useful info.

I cannot help with the question about unheadstamped cases, but the “AOP” on the primer is the Arkansas Ordnance Plant.