Mitsubishi J2M Raiden, part two

Raiden's performance improvement was focused on fitting the type with a turbo-supercharged engine. The first prototype of this variant was powered by the Mitsubishi Kasei 23hei (AKA Kasei 23a) radial engine rated at 1.820 hp, fitted with the company's own supercharger. The prototype used J2M6's fuselage with two cannons fitted aft of the cockpit. The 20 mm cannons fired upwards, in an oblique way, aimed at seventy degrees, just like the German Schräge Musik system. This version was intended to be used against B-29 bombers that operated at altitudes beyond reach of other Japanese fighters.

The second prototype of this version was powered with the same engine but it featured a different supercharger, this time made by Dai-Nijuichi Kaigun Kokuso (21st Aircraft Arsenal of the Navy) at Sasebo, Japan. This prototype was longer than its predecessor and was also equipped with modified engine cooling gills. It received the designation of J2M4kai Model 32kai and was converted by Yokosuka.

Only two prototypes were made of the J2M4 because of problems with the turbo-superchargers, rendering the mass manufacturing process impossible. According to documents captured by American troops, only one prototype was completed, while the other one was a regular J2M3 converted to a supercharger version.

The next version, J2M5 Model 33 was also going to be a high altitude fighter. This version was tested with a new Mitsubishi Kasei 26 engine (fitted in J2M2 airframes) rated at 1.820 hp. It's not possible to tell if the new cockpit frame and the additional flat profile of the upper fuselage in front of the windscreen was first adopted in this variant or in the J2M6, which was developed in parallel. The production variant, of which a total of 34 airplanes were made between May 1944 and August 1945 in Nagoya (though, according to other sources the total number was 43), was powered with the Mitsubishi Kasei 26ko engine fitted with a three-stage centrifugal supercharger. This feature made the J2M5 the fastest Raiden version as it reached a top speed of 615 km/h (382.143 mph) at 6.800 m (22.3097 feet) high. 
One of the J2M5 was shown in surviving photos armed with two 30 mm Type 5 cannons instead of the usual 20 mm ones.

It's also feasible that, due to limited availability of the Kasei 26 engine, some J2M5 airframes were fitted with the Kasei 23ko engine, making it similar-looking to the J2M6.

The last version, the J2M6 Model 31, looked on the outside like a combination of the J2M3 airframe with the J2M5 cockpit. This version was probably tested using the J2M2 airframe with the new cockpit and the flat fuselage top portion in front of the windscreen. Two prototypes were built between June 1944 and February 1945, but one was converted to the J2M4 standard. This variant appeared before the J2M4 and the J2M5, in spite of its numeration, and was developed in parallel, hence the usage of the Mitsubishi MK4C engine.

There were also some projected variants, that were never completed, like the J2M6ko Model 31ko, which was going to use the J2M3ko airframe combined with the J2M3 cockpit. The J2M7 Model 23 was under development and was going to have the J2M3 airframe, but powered by the Kasei 26ko engine. 

There are also mentions to a J2M8 variant which, apparently, was going to have a cut-down rear fuselage and an all-round vision canopy, similar to the of the A6M Zero. 

Sources:

1. https://en.wikipedia.org/wiki/Mitsubishi_J2M

2. Mushroom Model Magazine - Yellow Series 6110 - Mitsubishi j2M Raiden 'jack'

3. https://www.valka.cz/Micubisi-J2M-Raiden-Jack-t28908