Ross M-10 Military PLY marked
It would seem that Galen270 on "Canadiangunnutz.com" has come up with a very plausable piece of information on the PLY markings on the Ross M-10 .303 rifles. It was his inquiry that sparked the identification attempts two days ago.
He sent an e-mail to the Royal Marine Historian in England. The answer back was that the Royal Marines did indeed use the M-10 Ross and even Japanese Arasakas until the supply of SMLE rifles caught up.
The PLY indicates the Plymouth Division of the Royal Marine Infantry. Others are CH for Chatham and PO for Portsmouth Divisions.
This would make sense, considering that we have identified serial numbers from just over 1000 to just over 6000, at least 5000 rifles.
.