What Frank said is correct. In the original Stadium (1923), most of the seats were straight backed. The box seat area actually had folding chairs for years.
In the mid-1940's (I think 1943, but I am not positive) they replaced the folding chairs with the curved back seats.
During the 1970's remodel, they sold off the wodden seats, both curved and straight, for $7.50 and 5 empty Winston cigarrette packs (crush proof boxes, only!)
Here is one site that specializes in Stadium seats:
http://yankeestadiumseats.com/
This is also a copy of the Winstons for a seat ad, and a pic of my seat, which my sister bought me for my 19th birthday.