Karnak, our sources seam to be in conflict on the Radian Issue, My Francillon say on page 393 "In june 1944 the navy decided to adopt the faster Kawanishi Shinden as it's main interceptor fighter and continue Raiden production at reduced pace untill the Mitsubishi Reppu could be placed in production, howeaver this decision did not prevent the realization of two additional variants of the Raiden with better high-altitude preformance."
Karnak I do beleave that the J2M3a had 4 20mm Type 99 cannons but they were in underwing pods(2 of them) and only some 21 were built, the vast magority of the Radians that were produced having been the J2M3 at 260 being built, So I should think that If the Jack were modeled we would not see the J2M3a model, or the J2M5 model, but nthe run of the mill model the J2M3.