You can not really go by that. Waukesha could have made say 550 engine all this week and next Monday started on running 1800 engine. The engine number they quit with  and the first 1800 engine may have been just a couple of engine numbers apart and may not have been. I told you about my Notebook. We sold an 1800 C gas serial 137 094 844 Spec number2104 engine serial number 52202. The next 1800 we sold was a diesel serial number 141 652 spec number 2204. Engine number 59455. Notice there are about a difference of 7000 in the engine serial numbers and only a little over 4000 in the tractor serial number. There is less than a a difference of 3000 in the hydrapower drive serial number. What is your hydrapower serial number?