Hi Rick,
I believe you are correct. The tank in the picture I provided has the mortar capped. I did a bit more research and it looks like the T23 turrets manufactured by United Steel Castings, American Steel Foundry and Continental Hubbard (the latter 2 being used by Detroit Tank Arsenel who manufactured the M4A3) had the smoke mortar installed. However, Gary Binder mentioned back in 2006 on Missing Lynx that there was a Technical Bulletin from the U.S. War Department that called for the removel of the mortar
TB ORD FE43, dated 15 May 1945, entiltled;
"Tank Medium M4 Series and Tank Light M24: Removal of 2-Inch Smoke Mortar M3".
This order gives specific instructions on how to remove it. See here:
http://www.network54.com/Forum/47208/thread/1148101073/Sherman+75mm+Turret+bomb+thrower-+when-Perhaps the reason it is not on the Tasca Kit is due to them referencing post War tanks that had the mortar removed and covered over. Some people say that on some tanks it was done so well that you can't tell it was even there.
Nevertheless, for the period covered by my build (Dec 44-May 45) I believe the mortar should be there.
Here's a picture of preserved M4A3E8 in Korea. You can only slightly tell that the smoke mortar has been removed. Could easily be overlooked by a researcher as having never existed

At any rate thanks for mentioning this. It's great to get the feedback to allow me to build a more accurate kit. Now I just need to figure out how to add it.
BTW. I have the DML M4A3E8 Thunderbolt kit and the turret has the smoke mortar. Kind of dissapointing Tasca missed this given their kits are in some cases 30% more than the DML kits.
Cheers,
Dave.