I am pretty sure that Morgo 750cc kits had a different fin shape than the standard Triumph cylinder, they were shaped to match the head fins better.
Some other Triumph big-bore kits had various other fin shapes including closely replicating the standard fins, but they would have been cast at various foundries and have different casting numbers and marks than the originals. 800cc kits were also sold back in the day, so doing a CC check or actually measuring the bore is going to be your best bet.
In the end if the bike goes well it does not really matter, until you have to tear it down for a re-ring or overbore.
Throw up a photo of your bike for us to see if you can, just for kicks.