Denny
You are correct, IH was consistently inconsistent. The only casting on an Original with a date code is the transmission case I believe. The grill doesn't have one, the front axle or the rear axle tubes don't have one. The gear reduction housing may have, but I don't have one I can get to, to look. The grill has a cast P/N, and the axle tubes have a P/N on the flange (inset, not raised) and two thick front axles I looked at have some numbers faintly stamped into the back on the left side. They are stamped like the S/N's are, but so light I cannot make out the complete number on either axle.
As far as I know, all the production casting were poured at Louisville, at least until they went to the aluminum rear axle tubes and grill castings on the QL's an82 series. 409 had some 12 digit, 4 section, engineering part numbers cast on the axle tubes and reduction housings that basically told what the part was, the project, section it belonged too etc., but I don't know if they were cast in Louisville or someplace else.