Nandrolone Phenyl Proprionate. Without getting too fancy, Deca is Nandrolone Decanoate so they are both Nandrolone, just with different esters. The NPP is a faster acting ester with a much shorter half life. Most find that they can get very similar or the same results as Deca without all of the excess water retention and bloating of Deca.
If you already have Tren E though, that’s a very viable substitute. I don’t know how long you’ve been running the Test/Primo though. If you just started then the Tren E would be great. If you’re already a few weeks in, the faster esters of either Tren Ace or NPP would be the better choice to get them caught up and stable in your system.