I believe it is cast iron. Here are some more identification #'s, off frame, engine and trans, if these help. Thanx.

 

more pictures of identifiers

 

That is odd that the serial number is stamped on the transmission half bell, I too thought that stopped in '47. The '48 and later half bell actually doesn't have as much flat where the serial number is stamped, the access cover is a little closer to the outside edge of the bell making the flat area smaller. Maybe another clue that this is an early '48, since it also has a '47 or earlier steering wheel and steering shaft. Did any of the sixes have a cast half bell on the engine? All the six cylinder trucks that I have had had the stamped steel half bell.

 

I thought of that after I posted. I forget not everyone has the V8.

I agree some things on this truck look 47-ish, but I wonder if that is just "field replacements" after manufacture. His serial number is well into the year, in fact as I read it, may be near the end of '48. 30k units would have been a lot of trucks for a single plant.