[BCA]

Thanks for the comments, in particular to Tony for correcting the part number. My NOS positive ground voltage regulator has no markings or data plate to suggest the polarity - the only indication is from the packaging which gives the part number for positive ground model.
If you have an unmolested truck then the best indication of the original grounding would be from the holes sizes of the battery cable terminals.

Dodges: Both of my American 1/2-ton WC Dodges are missing the original battery cables so I follow the modern convention of using a negative ground: while the electrical systems charge OK, the ammeters read backwards indicating that they originally had positive grounding.
... Brian