Frank C - hey, 73 is 7hp 3 speed? Yes that is what a Model 73 has but it's not what the 73 designation indicates. Why are you trying to confuse our new member Ryan?
Shultzie - nice rearend on that 100. I see the really rare splined PTO.
Ryan - ok, so no one else posted info on identifying models by series. In your case the Model 73 is part of the 1x6/7 series. First off, 1x6/7 series includes the models 73, 106, 107, 126, 127 and 147. As you can see all the models end with 6 or 7. There was really no way to include 73 using this designation but we just know the 73 was the 7hp unit in the 106/7 series.
And to help with your understanding, the Model 72 (7hp tractor previous to your 73) is part of the 1x4/5 series which includes the models 72, 104, 105, 124, 125.
What Frank was sort of getting at is how the model number breaks down identifying the tractor itself. In your case the first digit 7 indicates 7hp and the 3 is just an updated digit from the previous 7hp model (the 72 as I noted above).
Cub Cadet models built from late 1963 until late 79 had 3 or 4 digit model numbers. The first 2 digits identified the engine hp. The last single digit or 2 digits (in the case of a 4 digit model) identify the transmission drive line. If the last digit or 2 digits are both even numbers the transmission is a manual shift - i.e. Model 106 is 10hp manual shift, and Model 1200 is a 12hp manual shift. If the last digit of a 3 digit model is an odd number then it's a hydrostatic trans - i.e. 169 is 16hp hydrostatic. If the last 2 digits of a 4 digit model start have an odd number 3rd digit then its a hydrostatic trans - i.e. Model 1450 is 14hp with a hydrostatic trans.
The 7hp tractors didn't fit this model numbering system really well except to designate the engine hp. The 70, 71, 72 and 73 all have 7hp engines with manual trans. The 86 and 800 both have 8hp engines with manual trans. IH didn't build a 7hp or 8hp with a hydrostatic trans.
When IH came out with the 82 series in late 1979 they stopped using this model numbering breakdown. We just refer to the 482, 582, 682, 782 and 982 as the 82 series.
That's my story and I'm sticking to it. I don't care what Frank says.