Definitely a 1973, model # 910021 was 73 only according to the Ariens literature.
Interesting....what other little changes do you observe between 73 and 74?IMHO it's a 1973. 910021 serial 001559 with Sno Thro 910995 serial 085151. The adjustment link for the friction wheel is different than the one on the 1974. Note: I have both: a 1973 and a 1974. I have compared.
I believe this chart has an error in that the 910006 is a 6hp model not 8hp.