Well, you have to be careful what people tell you at gun shows, especially if they have something to sell. In this case, he was right; HP38 comes off the same production line in St Marks, Florida that Win 231 does. Now that Hodgdon distributes Winchester powders you can see it in their loading data:
.44 Magnum
240 GR. LSWC CAST Winchester 231 .430" 1.620" 5.5 800 12,000 CUP 11.0 1334 38,100 CUP
240 GR. LSWC CAST Hodgdon HP-38 .430" 1.620" 5.5 800 12,000 CUP 11.0 1334 38,100 CUP
Some manuals show a difference, and I have seen it, too; but powder can vary lot-to-lot no matter what the label. I have some W231 in the old metal can that is noticeably slower burning than recent production HP38. I will use the W231 for .38 Specials as long as it lasts because my .38 press has fixed powder bushings for my standard loads. Changing to HP38 gives higher velocity than needed.