Grumman TBF and General Motors TBM Avenger

[ysi_maniac]

Avenger powered by Pratt & Whitney XR-4360-10 28 cylinder four-row

[jcf]

The R-4360-10 was a production engine, it didn't have an "X" designation.
Under the USAAC/USSAF/USN systems of the time "X" indicated "Experimental",
the Army also used "Y" for pre-production series test aircraft. 

The first military version, ordered by the USN, was designated XR-4360-2.

The P&W designation for the experimental Wasp Major-X Wasp, was TSB1-G.

A note about the standardized US military engine designations of the '30s-'40s, the dash
number indicated branch of service as well as engine series. Even numbers are US Navy
and odd numbers are US Army/USAAC/USAAF. This was closely adhered to until during
the war when the same engine subtype was being used by both services so they started
just using the same dash number to standardize the paperwork.

[ysi_maniac]

        

[Captain Canada]

That looks scary !