It is a typical civilian P53 - bought in and issued to both sides. The number on the barrel tang is a post-manufacture stamping. It was usually done for a rebuild/overhaul to identify the various parts of the rifle. These civvy P53s were not always 'to pattern' and parts were not always interchangable, hence the need to stamp the main components to ensure they could be reassembled.
A photo of the breech area would show proof marks and which area it originated in. Disassembly could also identify the gunmaker and even the contractors supplying the various components. Stock stamps that often escape are the (Confederate) stamp behind the rear of the trigger guard tang or the (Union) cartouches on the flat opposite the lock. Annother 'goodie' is the occasional stockmaker's stamp inside the ramrod slot!