Tolerances on silver coins were very tight and on both the dollar and trade dollar the tolerance was only .097 grams or 1.5 grains so the half gram difference in weight between the two is great enough that weighing the coins would most likely be able to identify a wrong planchet error.
A trade dollar on a Morgan dollar
blank would be an impossibility. A Morgan on a trade dollar planchet could be possible but highly unlikely. Why? Because there is no overlap in their production. Coinage of business strike trade dollars was ended by Treasury Secretary Sherman's order on Feb 22 1878. Morgan dollar
dies, and most likely planchets, were not ready until March 11th and the first pieces were not coined until March 13th (In Philadelphia, coining at San Francisco and Carson City would not occur until even later). So there were no light weigh planchets available before the end of the trade dollars, and there would have been ample time for rounding up and remelting any left over thrade dollar planchets before coinage of the Morgans began. About the only place you might find a trade dollar on a Morgan planchet would be on the 1879 to 1885 proofs. Especially the 84 and 85. Would they have run a special batch of heavier blanks just to make the proofs? The mintages for the 79 - 83 were small but large enough they probably would. But what about the 84 and 85? These were made AFTER the Trade dollar was discontinued and would they go to the trouble of making just ten heavy planchets in 1884 and five in 1885? Or would they just use Morgan blanks? I don't know, and I don't know if the weights of the known 84 and 85 trades are listed anywhere. (Possibly Breen encyclopedia but I doubt it.) On the other hand making the planchets might not have been that difficult. Just stop the rolling of a silver strip a pass or two early.