M45 is the 4 speed trans used in 240's.
M46 is the M45 box with an electrically controlled Laycock OD unit bolted to the back of it.
M47 is the M45 box with a 5th gear in a separate housing bolted to the back of it.
So you are both correct in a way. The M46 is what is actually called a 4speed + OD everywhere, and the M47 is the 5speed. M46 has 4 forward gears you can shift into, and the button for OD. M47 had 5 forward gears you can shift into.
M46 was used until the late 80's, and longer in Turbos. M47 started after the 86 body style sometime. M47 was never used from the factory in Turbos that I know of, which has lead to commonly accepted speculation that it is weaker.
I use MTL in my M46.
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1981 242 GLT-Turbo
1967.5 122S Automatic
2000 V70R AWD (shared)
Seattle, WA
www.psvcoa.org