The manufacturer appears to be United Aircraft Products (UAP). They subcontracted on many different planes. While UAP was based in Dayton , Ohio in 1940 they had merged with Aircraft Precision Products of Los Angeles. If this was in some sense a "standard" part, it may well be that the numbers are all UAP numbers and don't relate to a particular plane.
Update
Here is a link to a diagram showing the various hydraulic parts used in the P-38 cockpit area. Among the vendors listed, Bendix, Parker, Purolator and Vickers is United Aircraft Products.
http://legendsintheirowntime.com/LiT..._BW_p033_W.png
This does not show the hydraulic parts in other areas, particularly in the booms where I believe there were some flaps. At least we can say that UAP parts were in a P-38.