There was another dp chinese with a ji mark on it (吉）and it was for jilin province (吉林）http://en.wikipedia.org/wiki/Jilin
If this rifle is going along the same lines, it is also from that Jilin province but from a city abbreviation:
called Yanji city (延吉） http://en.wikipedia.org/wiki/Yanji
There are a lot of weapons factories in north east china (the 3 provinces near korea are all refered to as north eastern china), so might be onto something here with marked DPs coming from this region.
I put pinyin up for you if you dont have chinese character fonts installed, but if you have it installed, the characters should show up for you. Hope it helps, i can clarify some more later if you want, but plain and simple is it's another abbreviation marking.