AKMS - I’ll try to put this in an easy order.
Firstly, the Poongsan Metal Manufacturing Co. Ltd. had a commercial plant at Pusan, South Korea, at the same time the ROK Army was producing some ammunition at their own plant near Pusan, sometimes called the Pusan Arsenal, but actually located at Dongrae, which is also on the southeast side of South Korea.
The PMC commercial plant made ammunition with the “PS” headstamp, while the Dongrae (Pusan) Army plant made ammunition with the “K A” headstamp, which I assume means “Korean Army” in English.
In 1978, the Army Arsenal at Dongrae (Pusan) was transferred from the ROKA to the PMC Co.
At this time, in 1978, the K A headstamp ceases to be used. The very first headstamps on ammunition (.30 Carbine and .223) made at Dongrae under PMC control had an interesting headstamp, originally thought to be poorly stamped in Western Alphabet letters, but found to be actually in Korean ideographs that translate to “First Production Lot 1978.”
Subsequent production at the Dongrae plant under the control of PMC was headstamped "PSD"
to separate it from the commercial plant’s production, headstamped simply “P S.”
So, the summary is:
KA - Product of Dongrae (Pusan) Army Plant before 1978
PSD - Product of Dongrae (Pusan) plant after PMC took control in 1978
PS - Product of the original commercial factory at Pusan owned by PMC Ltd.
I am not sure when PMC first built or acquired their commercial factory. However, the earliest date of which I am aware that appears on PMC military-headstamped ammunition is “73” (1973) on .30-06.
I hope this sorts out the PMC question for you.