It's pretty easy to tell if you are in a 75S or not. If it's got 16 seats up front, then it's a 75S with D1 lie-flat's. As noted above, it was a swap from a 763ER to 75S which are both D1 aircraft. Also, last minute downgrades happen from time-to-time due to operational reasons. It's not like they sell D1 with intentional plans to downgrade them.
That's correct. It looks like it's N721TW, which is a D1 757.