Looks to be a Mansfield Alto with the short thank.
If it was installed before 1992, it may be 3.5 gallons.
If it's a 1.6, it will be marked that way.
An ADA toilet has an elongated bowl, and measures at least 17" to the top of the seat.
That means a bowl at 16.5" becomes ADA when the seat is added.
Tall bowls are becoming more popular, as the population ages.
Most builder grade toilets were in the $80 range.
There is no way a builder was going to drop any money upgrading toilets.
A new performance toilet will work better then what you have.
I'm not a big fan of the Mansfield trapway on that thing you have.