I have a L275, one hp less than yours. On mine if you look infront of the break peddles rivited to the trans there is a rivited on plate with the serial number, other info, and at the very botom is 1982. That happens to be the year mine was made (this could just be a coincident), but that number is there on mine.
When I, or anyone that I know, am looking at a used tractor that is more than a few years old what I am interested in is:
1) first look - from about 20 feet
2) close up examination (hopefully it has not been meticulously cleaned I want to see the grease and oil) - what is broken, dented, worn, modified etc
3) how many hours in it - are the hrs real
4) check all fluids and filters that I can
5) I start it and see how it runs , this includes some play time in the seat (no seat time no deal unless I can steal it for cheep parts price)
6) If it looks like it is going to be a go I will pressure wash it followed by more seat time and final inspection
7) Manufacture date if known
If you are buying a house it is location, location, location
If you are buying equipment or machinery it is condition, condition, condition