"...The 310 is a 1955 model, the original year of production, so there is no letter after the 310. The next model was the 310B. However, this plane was modified by Riley in 1963. the conversion was fairly extensive. First, the 470 c.i. Continental 240 hp carbureted engines were replaced with 540 cid fuel injected Lycomings that were further pumped with twin turbochargers on each engine. The hp at sea level is 295/side. Riley's book performance numbers may have been a little optimistic at 305 mph, 35000 service ceiling 17000 single engine, etc. I run the a/c without engaging the turbos (manual wastegates) and cruise at 200/210 at breathable altitudes. You may not have seen the a/c because from the end of May till mid Oct. my wife (Patti) and I spend our summers in Michigan at our lake cottage..."