YP is just the designation given to a pre production aircraft. XP is given to experimental and then it goes to YP. The way it works today for instance and I will use the Blackhawk only because that is the aircraft I know the best. When the protoypes came out they were designated XH-60's. They would build several of them and try differnet things on them. Then the pre productions came out and they were designated YH-60's. The YH's were given to the Army so they could do there own testing on them, then they inturn give there evaluations to the company that builds them and they make changes. When the final product enters production it loses the Y designation and became the UH-60A. The same was for aircraft in WW2 they were XP-80, then YP-80 and then finally P-80.