From: Hendaye, France
The B5M1 Mabel is Max Speed +2, Cruise Speed +58, Max Alt - 2,560, Climb - 290, Manouver +5, Endurance - 111 compared to the B5N2 Kate.
Normal and extended radius are the same for both planes. But the B5M1 uses the Mitsubishi Ha-33 engine, while the B5N2 uses the Nakajima Ha-35, which is the most-used fighter engine. There is a 65(0) factory producing the Ha-33 in Nagoya, and a 180(0) producing the Ha-35 in Tokyo at start. Without too many factories to switch around, easing the demand for fighter engines sounds to me like a good idea, because we need to replace Claudes and Nates in LARGE quantities.
At start there is neither B5M1, B5N1, or B5N2 production, There is only a 0(0) B5N2 factory in Hiroshima. Changing it to B5M1 and then expanding it would not cost more than just expanding it, or would it? So, I am inclined to use the B5M1 Mabel, and see if I can accelerate production of the B6N1 a couple of months... but I will think about it in November '42 or so. That will depend on the state of the Japanese economy, early-war losses, etc.
Just my 2 cents
I have some doubts about B5M1 Mabel data regarding Manouver in AE. IRL one of the key factors for the IJN to select B5N1 was a superior manouver of the latter after OfficialTests in November 1937. Furthermore it seems to me that favoring B5M1 is a big gamble regarding B5N1-2 outstanding results in 1941-1942.
Just my two cents
"If I go away to sea, I shall return a corpse awash, if duty calls me to the mountain, a verdant will be my pall, thus for the sake of the Emperor, I will not die peacefully at home...."