Rich,

I think the Inliners chart you looked at had a typo. They have another chart here that shows the 3837004 as a '55-'57 235. I found two other sites that agree.

The X suffix is for the regular production Thriftmaster 235 used in the 3100 and 3200 series trucks (w/ 3-speed).

Curt
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1953 Chevy 6400, 1957 Chevy 2dr Sedan
--"Mediocrity is easy, the good things take time"