what yr is my 273??

I have a 273 in my 65 barracuda and am curious how I can find out if that is the original motor. I found the numbers on the block under the heads on the front driver side, but they don't match any casting numbers I can find anywhere. Is there any way to tell or did they not put matching #'s? Is there a way to tell at least what yr it is? Thanks for any help..