Nick, the P boats were motor ships all built in the 1940's. This one has a longer name than Potaro so could be the Pilcomayo, just a guess. The E boats were built in the 50's and had more accommodation amidships.
PSNC had green boottopping RML had red, unable to discern the colour in this photo. Looking at the accomodation layout I suggest she is a RML vessel certainly not the Essequibo as I sailed on her but am not an expert on their vessels, so its over to the RML officinados
I also think it is the Pilcomayo I was in her but I,m only going by the length of the name on the stern although one of the other P boats was the Paraguay , I bet the lowering of the jumbo and the topmast,s was a bundle of laughs best regards Dave .