Look in this area for year letter and numbers, as in: A273 will be '65, B273 will be '66 and C273 will be '67, but as the others have pointed out, it appears to be '66 or '67 vintage. Check the other side of the block for a casting date which would narrow down the year application.
The 42303 number is probably a rebuilder number.
View attachment 1715154042
