There were several 'leftover' 1800 gas tractors that were updated and made into 1850s. I have a copy of a note from the factory where 1800 serial 139 269 was re-worked to be an 1850 and given the serial number 157 192, which is after your tractor. I'd bet that the same thing was done to your tractor. You can get a build card for it by calling the Floyd County Museum with your serial number at 641-228-1099. It should have some kind of notation if it was a converted tractor.