Do you mean Type 55? I don't see a Type 51 on the Polaroid site. Type 55 is 4x5 sheet film that gives both a positive image and a negative. To get the best results for each you should expose for the one you want. If I recall correctly I expose at EI40 for the best negative exposure. The negs were beautiful although the edges had to be trimmed to fit into the 4x5 holder for the enlarger I was using. I stopped using the film because about 80% of the negs were ruined because I couldn't get the film out of my 545 holder undeveloped (I wanted to put it back in later and develop when I had a clearing tank handy. I don't relish carrying around a big bucket with all the other things it takes to shoot 4x5). Every time I pressed the release switch and pulled out the film I just got the cover, the film stuck in the holder and was exposed by light leaking in where the cover used to be. I don't have this problem with any other kind of film except Type 55. Comes out fine when I push down the 'develop' switch and pull it out.
The emulsion is also a bit soft, so unless you treat it with a hardener (directions on the web, run a google search) treat it carefully.