05 21 is the build date, May 21st. The number 1 looks like the letter I on Chevy engine stamps. GF was used for 2bbl 283s in full size cars in the mid 60s, probably a 1965 engine with that May 17 1965 casting date code
Thank you Squirrel! I had trouble uploading the date code, here it is. I think it is a five but I thought the 1 was an I!
either 5 or 6? probably doesn't really matter which. The I thing tricks a lot of guys who haven't looked at a lot of numbers