CAT Twenty and Thirty

I called on a recent ad for CAT twenty and thirty. They are located really close to me and I can't resist the opportunity to go look. I tried to access the Archive information on our home page and it isn't working. Can someone help with some basic information on the twenty and thirty like year, serial number and where built? Are there any critical areas or items I should look out for when inspecting these old crawlers?
Thanks for all your help.
Jerome Pasto

http://www.westernfarmcollectables.com/specs.html

Thanks for the help.
Jerome