White Ensign Models has it all.
http://www.whiteensignmodels.com/acatalog/index.html
Fret 1
Fret 2
The 50s are on this fret. They include a complete carriage system (almost functional). The gun sides are PE appliques and roll the barrel from the perforated piece
37mm AT gun
37mm Oldsmobile Cannon
20mm Oerlikon on pedestal base
This is for the Revell Gato. The 20mm which is included in the PT-109 kit is pretty good, and WEM's PE set has detail items to improve upon it. This is not necessary unless you have excess money burning a hole in your pocket.
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Edit
oops! forgot the roll-off torpedo racks. These come with resin Mk13 torpedos
