That is a nice Allis Chalmers you have there!! the 3/4's have a pretty stiff ride... many dont like it... maybe you should send yours my way.. . : )
by the looks of it yours may be an early 400 the seat changed later... that seat is similar to the 300 deluxe seat that i have seen on other early 4's... would you be willing to share the serial # I sorta keep track of this stuff..
BTW, you'll want to pay special attention to the mower deck around the front near where the brackets bolt to the deck shell... they all rot right there if not properly taken care of!! (actually, on the left side of the bracket is the worst ... grass sits there...)