I just purchased an Allis Chalmers dozer that has HD6 on the front part of the crawler. The person that I bought it from says he thinks it is actually an HD5 with the front end from an HD6. How can I tell for sure and where can I find the serial number. It has a 271 Detroit that has 63 in the casting of the block. I think this might be the year that it was built since I also own a GM bus with a 671 and it has 58 embossed on the block which I am pretty sure is the year it was built. Any help would be great. I want to order the books for it and want to be sure to get the right ones.