This is one of the cases that Section 500.8(A)(3) was written for.
The ruling text is in Section 502.10(B)(4), “Boxes and Fittings. All boxes and fittings shall be dusttight.” While dusttight is a defined term (in two places as a matter of fact - an NFPA Manual of Style no-no), it isn’t defined too rigorously. Conduit body is also defined, but it doesn’t exempt a conduit body from being a “box.”
You won’t find any conduit body listed specifically for Class II, Division 2 or as dusttight –because they don’t need to be. Conduit bodies listed for Class II are dust-ignitionproof which is gross overkill for dusttight.
Personally, I wouldn’t hesitate to use either Form 7 or Mark 9 in Class II, Division 2. If I were REAL concerned, I might tuck in a gasket, but I probably wouldn’t. And before anyone cautions me about using gaskets in Classified locations, don’t bother since there is no attempt to maintain either an explosionproof (which doesn’t apply) or dust-ignitionproof rating in Class II, Division 2. Besides, that general prohibition has been gone a long time as long as the gasket/O-ring doesn’t interfere with the intended flame-path.
Robert B. Alexander, P.E.
Answers based on 2014 NEC unless otherwise noted.