If you have the "U" belts, without no "L" in the back, they will slip right in. These are regular RPD "U" link belts, made to feed 7.62x39, but they fit VERY tight. I would think an RPD has a very strong recoil spring and/or else a heavy BC, to strip rds from those tight belts. Frankly, I don't own one so I wouldn't know.
If you try inserting a 7.62x25 rd the normal way, they will fall out; if you insert them backwards, they click right in & click right out. Easily. Surely someone else here has tried it by now, and can verify that.