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Wasn't the M40 & the TH400 one in the same?
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Yes. M40 was the ordering code for a TH400 trans. Same code used for all TH400's.
In 69 (not sure on other years), M40 was also the ordering code used for the TH350's and showed up on the window sticker. M38 was actually the internal GM trans code for TH350's. Go figure....